My dirt base is lifting off the base after the glue dried. This happened to me before but I assumed it was because I had watered down the school glue and it contracted too much. This time I just slathered the whole top of the base in glue and threw the dirt on, shook off the excess, and wiped the rim. Is this just a normal thing for using PVA glue to make a real dirt base or am I missing something here?