I'm in the process of painting the interior of a house that my family just moved into. It's going slower than I anticipated. One problem I'm having is that the roller cover keeps walking off the roller. I'm using good quality tools but it just won't stay in place. I have had this problem every time I have painted with a roller. Any suggestions for how to prevent this? It is a bit of a hassle to have to nudge the roller cover back in place every few minutes.