assume(p > 0, a > 0);
=> "Is psey positive, negative, or zero?"
PSEY is an internal variable, should not be visible to user.
See comments in src/hypgeo.lisp about PSEY.
From the comments, PSEY represents the parameter in a Laplace transform. Maybe it will resolve the problem to do assume(psey > 0) or something like that. Ideally that assumption would be attached to the result (after substituting the original parameter symbol for PSEY).