I have a similar pump that I use on my white water raft, it will sometimes develop a sticky check valve and refuse to pump. I can usually fix it by running some clean water through the system.

Also you might make sure you have the hoses on the correct fittings. Mine has both a 'suck' and a 'blow' connection. I have been fooled when trying to top off the boat only to find I had it connected to the 'suck' end!