I is probably a little of both. If you do not have enough pressure, and there is a bit of humidity in the sublicloth, when you press it, it is puckering a bit and the steam from the humidity is carrying the sublimation dyes off to the side of your image as it escapes. I would force dry your sublicloth under your press without closing it, and also make sure your transfer paper is dry the same way. Use a good medium or a bit more pressure to make sure the sublicloth stays flat when heating.
