Also, the romanian Mig-21 are converted to NATO specs, the ukrainean are not able to use them without training. Maybe it is the same issue with the Polish/Bulgarian Mig-29.
It seems like Europe announced the plane transfer without enough consultation from the countries who would actually have the planes needed. At least, that's the impression I've gotten from how fast most of them denied any MIG loan to Ukraine after the initial announcement was made
Apparently there are a number of technical challenges, mostly related to the pilots not being up to speed with the weapons conversions, ensuring they are not identified as NATO planes and the time required for training. Someone else pointed out that it may also be the case that through channels Russia gave some kind of ultimatum about this, but that is pure speculation.