Did you check the purity of the free MDMA? Could there be somthing in the prepeartion that is reactive (and destroying) towards the immobilised BSA-MDMA?
If you immobilise the anti MDMA antibody you have an interaction with the BSA-MDMA. Can you block the interaction with the MDMA alone? For instance:
1) anti MDMA (immob) + BSA-MDMA = response
then regenerate
2a) anti MDMA (immob) + MDMA = low response? due to low Mw
no regeneration
2b) inject BSA-MDMA again = no/lower response: MDMA is blocking or same response as in 1 = MDMA is not working