小中大
I think the possible reason is 加上糖链后对分子量的影响不足以达到通过电泳看出来. You could use FTICR-MS or orbitrap-MS to measure accurate molecular weight of the reduced form. If you can not access these high resolution mass spectrometers, you also could digest the target protein and original protein using a enzyme, such as trypsin, then analyze resulting peptides using a low resolution MS to see if there is any differences between them, then verify the difference is glycan. Or you can run a 2D gel, glycoproteins may have different pI.
Anyway, good luck.