My nice long Zeagle is about 5" wide and the actual passageway through the seals no more than 1"
It has a LP inflation thing-a-mi-wotsit but it can also be successfully inflated with a regulator. I know 'cos I've done it.
However, with it being so narrow with a tight air access, in a current the bag bends and easily constricts the passageway
I suspect that may be the problem you're having and if you can just get a wee bit in to hold it slightly erect you should be able to continue adding gas just fine
