floor filter prob

i have a floor filter i just got and i filled the tank half way up. i will add gravela nd rock today which will displace the rest of the water. now, i turn the bottom filter thing on and the pump part at the top doesn't pump anything.. what could be a problem? does the top have to be covered by water? it's only half way up like i said, so what's the dealio?

there is a silicon seal to keep the piping together, but i don't seen any water being pulled from there, plus it shoudln't affect the problem i have.

oh and it is making noise but the water level in the tube isn't being sucked up. i tried syphoning but i was unsuccessful.

ok i did the deed... i sucked straight from the filter. it was gross but it worked. i hope a little bit of saliva is ok?