I've noticed that sometimes after i cover my cakes an hour later the fondant is sagging and/wrinkled on the sides. Do you know what causes this?

My steps:

dam and fill, crumb coat, frost cake, insert stick in middle to vent the cake, refrigerate, cover cake

Hard to say w/o a pic but from your description it sounds like your fondant may be too thick and the weight of it is causing it to stretch and sag.  How thick are you rolling it?

If it's only doing it around the area where the filling is- then you may have too much filling between your layers or your cake is not settled enough.   Try letting the cake settle at room temp with just a crumb coat and the vent, then frost and fridge.

Did you refrigerate the crumb coated cake before applying the fondant?

I have also noticed the sagging and it is due to the change in recipes that now contain palm fat (at least in the UK)  which dissolves in warm temperatures. In fact the ready-made fondants have never been so dismal! You might want to try using a bit more icing sugar and chilling your fondant prior to rolling. I can't guarantee it will work as I haven't tried this idea, yet.

Does it look like the above picture? Or is it more like sagging and then tearing? The above is called Elephant Skin. Usually caused by dry fondant. Now you may not think it was dry when you covered the cake but you may have accidently done it when kneading the fondant and adding perhaps cornstarch or powdered sugar? And then doing the same to roll the fondant. Maybe you rolled and then started over and kneaded all that powder back into the fondant. If it's not too bad, you can fix it by making a paste with fondant of that color and a couple drops of water. Microwave it just till melted and cover up the wrinkles as best as possible with a small spatula.

http://ericaobrien.com/blog/gunge-its-a-miracle/

Thanks for the responses.  Picture above.

Looks like that cake has sunk, probably was too heavy to support its weight and the sponge was too soft? Because I can see the fondant on the bottom and I cannot think that you left it there on purpose, so I assume you trimmed it properly after application and then it sagged. Was your buttercream too runny? could it have melted into the cake thus compressing the cake layers? Did you support the top layer with 4 dowelling rods and a thin cake board? If not, then that is the reason it sags.

Yeah, it looks like you have some bulging on that top tier which would indicate you have too much filling for the consistency and your cake is settling.  The weight of the cake is pushing down and pushing the filling out the sides.

Although, it's hard to tell from that angle but it looks like the bottom tier does not have bulging so much as the fondant is just sagging.

Sooooo, maybe we can narrow it down...

1.  what are you using for a filling?

2.  How thick are you rolling your fondant?

3.  What are you using to support that 2nd tier?

maybe one of us can help you out if we know that info :)

This is definitely from the cake settling.  The filling between layers may be too soft or melting.  Needs more support in bottom cake to support weight of top cake. Fondant may be rolled to thick, contributing even more weight to the cake. Also, that looks like an air bubble formed under the white fondant between the two blue stripes. Prevent air pockets by smoothing fondant completely on top before starting on sides, always smoothing from top to bottom.  If you still get an air bubble a few hours after covering your cake, pierce with a sterilized needle and gently press the air out  with your fingers. Spackle hole with MinaBakes' recipe or cover with a decoration. 

I hope this has covered all the possibilities.

So you guys suggest I use less filling and thinner fondant? OK great. I'll try that this week. I use the large plastic dowels in my cakes and place at least 4 in the bottom tier.

Yes, make sure your fondant is somewhere between 1/8 & 1/4 inch thick.  Let your cake settle at room temp as well.  You sound like your using a good support in the cake.

Let us know how it goes  :)

Thanks for all your help!

You didn't say if you have used a cake board between the two layers of cake...

yes there's a cake board underneath the top tier

Here's another pic the wrinkling that happens. This cake  thin fondant and 4 plastic dowels.

That looks like your fondant is very thin  

Oh, goodness, it does look too thin now.  Your fondant should be between 1/8 and 1/4 of an inch.  What are you using for fondant?  

The good news is, I don't see any bulging there so your filling is good :)

I'm wondering if the surface of your iced cake was not sticky enough. I had one turn out like that when I forgot to moisten the surface of my crusting icing.  The fondant didn't adhere to the entire cake. Once I moistened the buttercream, the fondant smoother got all the wrinkles out. 

What kind of fondant are you using?   I always seem to have this problem with homemade marshmallow fondant but not with Fondarific, my fondant of choice.   Your fondant could be the problem or Iike the others said, your cake is too light weight to hold the fondant.  I've made that mistake before too.   You might need a denser cake.  Hard to say which one just looking at pictures.