Pictures?

Without seeing it, I can't be certain, but I would most likely take them up. Those sheets of tile are sometimes out of square and individual tiles need to be reset - is that, by chance, where the problem started? Marking a grid on the floor before tiling in future will make an out-of-square situation with those mosaic sheets more immediately obvious.

https://picasaweb.google.com/lh/photo/W_oweXXZLpA4Uf-CPFSw7rfwg-GKVGol_337XDi5fAY?feat=directlink

I think you might be right about the listello - I did start with a grid - just forgot to close it off - um, duh! it's my first floor - won't make that mistake again! The only problem is that the mortar is dry and has been for quite some time. Does that change your opinion about pulling it up?

I agree. Pull it and redo that part of it. The mortar, unless it's been down for a month or more, shouldn't be all that hard. Once the tile is up, you should be able to take the thinset up (at most) with a hammer and painters 5 in 1 tool. You'll be much happier in the long run.

Cool - I always thought those things were a gimmick, but just read a review of the 5 in 1 tool and am going to pick one up tomorrow. My putty knife is my favorite "tool" now so I can see this quickly replacing it.

The mortar under the tile has been down for about 3 wks, so close, but the thinset underneath that has been down for a month - I had to cover the heating mat first, let that dry and then lay the tile.

Thanks for the advice - I've been pulling my hair out!