Don't use more than one popup at a time, or this is what you'll get. Instead, hide the earlier form if there is an order like A - B - C then backwards close C, show B, close B show A. A subsequent form can also inherit a size based on the previously opened form (e.g. maximized or restored) but not usually if they're popups. The main problem with Access and popups is that Access doesn't deal with the Z order of windows. The effort required to work with the form window handles isn't worth it IMHO, but you could research API calls for window handles, but I wouldn't bother.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.