my problem is that when I use stacked construction, the tiers often start to sink in - am I not using strong enough dowels between the tiers? Or is my cake too soft? Any help would be most appreciated.
If youur cake is starting to sink something is wrong. You should not have the cake sitting on the cake below without a good support system. Check your construction make sure your dowels or system pillars are even and cover enough space to support the next tier up. You need to have a plate, or many boards depending on the number of tiers above to be able to support the cake on the doewls.
There is a system from Bakery craft that is reasonable in price and offers total support and stabilbilty for your cake for both transport and just staying up.
I think there is a tutorial for this sps system if not check you tube on construction of a tiered cake. I know I have seen it.. in fact I think there is a folder on the front page that shows the sps system.
If your tiers are sinking, your supports are probably too short. The SPS system by Bakery Crafts is a good system. If you cannot get that, using Wilton's hidden pillars and plastic plates will also work. Just make sure the supports are not sunk too far down into the cake. I prefer to have my dowels just slightly above the top of the frosting. The key word being slightly. Too high and the cake will wobble.