There's a couple things that could be causing this problem.
With regard to priming, Reflex1 hit the nail on the head... the 405 needs to be filled up with 2gal [IIRC] of water before reattaching your intake and output hoses. The seal is also very important, if it is letting any air in, you'll have experience symptoms you're seeing.
A member of the forum on cichlid-forums.com suggests this:
Originally Posted by Rockydog
Whenever you fill or refill your 405 you should rock it and purposefully bump it on the bottom to dislodge air. Doing this 4 or 5 times in the first hour will expel 99% of the air. It will come out in clouds of bubbles. If you don't you will get a few bubbles once in a while for days after.
Every time the Fluval 405 loses power (even momentarily) it will go through its start-up cycle. What happens during this start-up cycle is the 405 starts pumping for 1 minutes - stops - then pumps any remaining air out.
I'm wondering if you might be having some power issues. An el-cheapo UPS from BestBuy would probably fix (or prevent any future) blips in your power.
Double check all your hoses and check for any hairline cracks and/or small leaks. Pay special attention to the fittings. Replace
anything that is cracked, don't just silicone and hope for the best.
Did this start after a scheduled maintenance?
If so, be sure to confirm that all seals and attachments are in good condition, firmly seated, properly lubricated (where applicable).
Were there any lightning storms, power surges, or power outages in your area recently?
If so, this event may have damaged the electrical components of your pump and may need to be replaced.
Have you ever used vaseline on the seals?
If so, I would look for a product that does not contain petroleum. I know from other hobbies that petroleum can cause rubber to degrade and no longer achieve a proper seal.
Hope this helps, let us know what you find out.