I have had a hard time filling the FW tank. Still do occasionally.
I also did as HopHed, my inserted hose is about 2' in length. This seems to help. What I found to work best was to remove my vent hose and blow in it.
With mine I think the vent hose is lower at some point allowing water to enter and create a blockage so the air cannot exit. When I blow on it I can hear water gurgling then a push of air will escape through the vent hose. I'll do this a couple of times and seems to fill up completely.
Bit of a nuisance but it fills and I'm not going underneath to fix it.
John