Hi,
What i can assume is your tail transistor could go to triode region which will effect your gm, gain, thus the output signal is not amplified accordingly and unbalanced. Is that for all the range of input swing (small to large) your amplifier gives unbalance out. If only for large swing it gives unbalance swing, then it could be because of the tail transistor working in triode regime, but if it didnt work for both range, then could be other factor.
Regards,
Suria