The answer is a little late, but in my case the cause was air in the system. I injected water into the in-port with a syringe until water came out of the out-port. This basically flushed the system. The Aquabloom solar pump then pumped water again.
No guarantee, of course, but it helped me.
Good luck.

Have you checked to be sure the pump works with just the input and output tubes? If not, perhaps it is the motor that is dysfunctional. I did not have this problem.

Possibly input and output mixed up? Is the intake filter clean?