JAVA--I imported method Graphics.drawpolygon(para1,2,3… I can't change it during runtime though?

By itself, it displays my polygon. However, if I at ANY point in my code-- even code that happens after the polygon should be made-- attempts to change any one of the two arrays that make up the polygon, it will not display--at all. In fact, it will not display unless I maximise the window, after the code that changes it has already run. If I need to give a code example, I will, but please, this is really confusing me??