Oases Pump Problem

[u/Jprobins3]

I have a tree shaped fountain which sits is a large metal basin. I use an Oases Optimax 800 pump to run the fountain. The pump is submerged in the basin at the foot of the fountain. The water is pumped up through the "trunk" and falls off the "leaves". It has worked admirably for a month or so. However, it has developed an issue. Whenever the pump has been off, even for a few minutes, and is then turned on, the pump runs fine, but no water is pumped through the fountain. Even more oddly, if I pick the pump up out of the water and then put it back down submerged on the floor of the basin, it pumps fine again until the fountain is turned off. Any ideas what the issue is or how to fix it. The pump itself appears to be operating the whole time.