Welcome to TFP!
When heating the spa you need the shared side of the plumbing set to spa only, on both the suction and return side. Normally the spa plumbing lines are fairly short and get better water flow than the pool side.
I would start by checking that you are getting a strong return flow at the spa, best to try this with the spa jet pump off so it is obvious which return is coming from the heater. If not there is something blocking the flow, perhaps as simple as a full pump strainer basket or needing to clean/backwash the filter, perhaps something more complex.
If you do have a nice strong flow, perhaps the heater is failing to sense the flow properly.
Also, do you have a heater bypass? If so too much water might be flowing through the bypass.
A photo of the equipment pad, and your filter pressure in both pool mode, and spa mode, will help us diagnose further.