When using Aprons, if you overlay one with another you need adjust the priority. So if the apron is drawn below another and you want it to be on top, make sure you check “Force draw above the runway” in the property window while also changing the priority. So, if you have three layers, the bottom should have a 0 priority, the middle a +1 priority and the top a +2 priority. Now if you add another layer but want it just below the top layer, you need to drop the priority down by clicking the minus button.
Play around with it and you get the hang of it.