I'd say that the mask is probably not the right size (sounds like it might be too small) if she's getting sores... You want to be able to get a finger or two under the edges of the mask when it's on, maybe you already know that. =)
Does it have that fleece edging stuff? If not, you could probably sew some on and that might take care of the whole issue.
I've only just started using a flymask on Lacey myself so there are definitely people who are wiser about this than me. haha
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.