hps is the one that requires a "jolt" to start, isnt it? (may be mh though...).

if so, maybe your ballasts isnt firing hard enough to btidge the electrical gap.
check that the bulb is a snug fit all the way into the socket. maybe a slightly larger conversion bulb spread the contacts or something?

ive got no clue, this is my 2c (1c with conversion rates)