Did you run a non deglycosylated control to make sure that your glycosidase is active? Did you try a different/fresh kit/enzyme? Can you rule out protein degradation?

SE Hplc looks fine which says that protein is not degraded.

Ya non-deglyco sample have multiple peaks but at different mol wt not matching with deglyco sample, which says that glycol sample can`t ionise full.

kit arrived in cool packs and it is working for other antibodies.

Are you sure that it's actually N-linked? If your kit doesn't touch O-linked glycans, I would run your degly heavy chain on an intact MS method (C4, 10 min gradient 15-45% B should do it). If you have a peak that's + 203, +162, it could be O-linked, Core 1 or Core 2. O-linked is very difficult to remove in some locations.

If that doesn't work, try adding some denaturant to open up the structure and let it breathe a little. The glycans might be internalizing, which would preclude access to any enzymes. Rapid PNGase F has a buffer that does that and it's MS compatible. Stay away from RapiGest (Waters) though, it breaks down into PEG upon acidification.