I have a Charles Daly Field pump. After firing, I pump it and a lot of the time it doesn't want to put another shell in the chamber, it just sticks in the tube unless I pump multiple times or push the shells back and forth in the tube. Is something sticking?
If you haven't already, pull the spring out of the tubular magazine and check to see that it is not in a completely compressed state. Often a waterfowl plug gets jammed in the magazine and the spring loses it's ability to uncoil. In this state the spring cannot push against the shot shells in the tubular magazine hindering it's ability to feed the round into the action.
I suggest you could take the shotgun to a gunshop and ask them to check it.
You might want to make sure that the shotgun has been maintained and been cleaned properly, and that the pump has absolutey no problems with it [Apparently some bloke tried to pump a shotgun and the entire thing fell apart. Apparently there was so much dust and rust around the pump that it was a miracle that the the entire thing didnt blow up in his face].