Got 2 used JM Platinum reels, one 8.3 and one 6.8. The 8.3 casts like a dream, however, the 6.8 doesn't seem to cast at all. Have same settings on both but just seems like the spool stops on it's own about 15-20 yds into the cast and the bait just falls out of the sky. 

 

Don't think its a tension issue as I have it set the same (same lure and rod. Could it be internal? Or is there anything I can lube or clean out that might  help out?

 

First cut off your bait, wind in the line, then put some tape on the spool. Back off the spool tension and completely back off your brakes. Hold the thumb bar down and spin the spool with your finger. If it spins freely and continues to spin then it isn't bearing related. If it won't spin or only rotates for a revolution or two then it's probably spool bearing related. If so pull the the side plate and check the bearing. Cut half the swab off a Q-tip and insert it. Then rotate the inner race. Feel for resistance. Then pull the spool, wipe any excess grease off of the long shaft if it's present, then check its bearing by rotating it with the edge of your finger. If they're sluggish or worse, clean and lube each one. 

Sounds like a good cleaning or at least bearing flush is in order. Check for line digging in and that the rod is appropriate for the weight casting