1997 Jeep Cherokee Won't start with new battery and starter

I recently replaced the battery and starter/selonoid, on my '97 Grand Cherokee, because it would not turn over. A few months later it would not turn over again, totally dead not even a click. I returned the starter thinking it was defective, installed the replacement and all was well. Yesterday, three weeks after installing the replacement starter, it would not turn over. I removed the starter, spun it with jumper cables, reinstalled it and the car started. I know something else is wrong, but don't know what to check next.