More info needed.
Hord to say what the problem is without seeing it.
Do you mean it rolls out when rolled by itself or with the shaft attached?
If it only rolls off with the shaft attached, it may just need the joint re-faced. Quick easy job.
If it rolls off when rolled by itself, does the SS joint appear to be on crooked or does the forearn appear to be warped?
This would be a bigger problem. Replacing a faulty SS joint will cost a little more, but is doable. If the forearm is warped, your into a much bigger can of worms. If the warp is at the A-joint, you may be able to cut and re-join the handle to the forearm and correct the problem. If the forearm itself is warped, you're in real trouble. Could possibly be turned to remove the warp, if you don't mind a thinner cue (and the points would probably not be even when you're done). Or, you could replace the forearm entirely, but you might as well buy another cue at that point.