The Ukrainian troops in Debaltseve were encircled when Minsk II was signed. The separatists and Putin already basically said they weren't going to let this one go, but I guess everybody pretended they didn't hear that and so they just did not mention it in the agreement.

This is a problem right? No side wants to give up land. If the fighting had stopped there, that would have created a little exclave full of starving Ukrainian soldiers. How do you deal with this exactly, without talking about it? One side has to give in. Either Ukraine withdraws or the separatists withdraw from the encirclement. Since the separatists were winning there already, they forced the Ukrainians out.

Fighting did die down everywhere else, and there as well once the Ukrainian troops broke out. I guess the Ukrainians were hoping they would get to keep it if they hold on to it by their fingernails. In any case, this isn't the main reason the agreement tanked.