So I have been doing cakes for 2 years now and I feel like I have achieved some big things I thought I would never do. Such as gum paste flowers, chocolate work, etc... But the thing I thought would be easiest has become quite a problem and concern for me. The past two tiered cakes I have done have either collapsed or had a huge lean to them. I have watched, read and researched until my eyes hurt on how to stack a cake. What could I possibly be doing wrong?

The last cake was 3-tiered, all banana cake with strawberry cream cheese filling, covered in fondant and all fondant decorations. I used cake boards to separate the tiers, along with plastic dowels and even a wooden dowel through out.

Any suggestions would be appreciated.

Without seeing pix of your 'leaning towers of Pizza" it is hard to say.
Here are some things to consider:
were the layers perfectly leveled before putting together?
Were *ALL* of the dowels cut to the *same* heigth?
Are you sure you pushed them in *straight*?
Once the support doweling (whatever type used) was in did you put a cake circle on & check again to see if it was level before adding the next tier?
was the table it was displayed on level?
did someone bump the table once the cake was on it?

Those are some things that can cause a cake to fall.

SPS is the support system you need. It's inexpensive and easy to use. Your cakes won't collapse.

Not sure if this makes a difference but, I also make sure the table is completely level before setting up a cake. I bring a level along with me. if the table is not level I will try to level it by putting a little piece of cardboard paper under the leg.