Ok, I have seen this problem all over the place and followed many solutions but not sure what our issue is. It may be the check valve but my thinking is that its not based on my testing.
If you look at the pics below the first pic shows suction side. Left pipe to the 3 way valve is the spa drain. Second pic shows returns. Very top pipe above 3 way is the spa jets. Very bottom pipe is fountain in spa in which line has a check valve. We have a fountain pvc pipe in the spa just to be clear. I have checked both 3 way valves and they look in good order. A pool tech also checked them and agreed they looked good.
We performed a test to try and rule out the check valve by turning the ball valve off for the fountain since it contains the check valve. The spa still drained down to the jets. It takes about 4-5 days for the spa to drain all the way to the jets. I dont want to just start changing check valves etc and then still have the issue.