blowers issue when filter is running

Okay have a poll with a half moon spa on the side of the pool (when blower is working water will splash and fall into the pool). When I run my filter, 4 blowers will be working (3 in the pool and one in the spa). The 3 in the pool work perfectly, the one in the spa is a problem - Somedays it works, somedays it works but it is very weak and somedays it does not work at all (i.e. no water coming out). The last few days nothing is coming out - I tested it today by putting the filter off and then on - I bubble or two comes out of the blower in the spa and that is it (I do get bubbles coming out of the blowers in the pool as well but the water flow is perfect).
Anyone got any ideas what is going on here?
Thanks so much
Gary

Have you tested the line going from the pump to the spa for cracks and blockages?

Last night I swtiched the pump valve to spa with the filter on and the water came flowing strong through the spa blower so there is defintely nothing wrong with it. I have it always normnally set on pool and it has always had water come out both spa and pool blower. Could it have anything to do with the thermostat on the filter / it almost seems (and I am speculating) that is a power strength thing where the pool blowers are dominating and all the power is coming through there and little power for the spa??