Not a lot to do but speculate with as little as you've given. Three most popular:
1) Differential absorption. Spalted wood, end grain, things like that suck in more juice than face grain.
2) Application errors. Get the flashlight out to check and see if you have missed spots with your rag or brush.
3) Inadequate surface preparation. The stuff wants "tooth" to stick, and if you're not using that same flashlight to make sure your paper or wool has dulled the old surface, it can pull away from a slick area by surface tension.
Any of these should have obvious appearances related to their cause.