A couple of quick thoughts:
When you make a MAb hybridoma, you are doubling the amount of chromosomes
in the cell, and the cell wishes to become diploid once again. Hence, it
degrades or spits out the chromosomes it doesn't need (in this case, the
genetic info needed to produce your antibody).
1) Try a different immunization protocol. You may need to immunize longer
and more frequently to get a higher titer.
2) I do not know how you are attaching the hapten to the carrier protein.
There may be insufficient epitope present to generate the specific
response desired, and the mouse is reacting more to your carrier rather
than to your epitope. By increasing the target immunogen on the carrier
(if possible) you may get a better response.
3) You may want to use an adjuvant of some type to boost the response.