I tried the Wellbutrin and Buspar combo and the Buspar was not strong enough to keep the anxiety down when on Wellbutrin. I now take the Buspar and it does work well for the anxiety without any side effects to speak of. Wellbutrin I stopped.
I would go with an increase on your Buspar if you Doctor is willing. I would guess you are not at the max dose on the Buspar. JMHO and each person is different in how they tolerate a medication.
It kicked up my anxiety as it has no antianxiety properties like many of the ADs do. I started the Buspar at the same time but still felt anxious on the Wellbutrin. I do like the Buspar and I have GERD too.