Maybe you are already doing this but have you tried double clutching this gear? If you are double clutching it might be a problem with your tranny. Maybe just an adjustment is needed. You might need to put in a call to your mechanic and get his/her opinion. Good Luck

Splitter is used for the gears 6th and up ,, down for 5th thru 1st

I don't think he's talking about using the splitter again (At least I hope not) If he were to split down he would be pushing the splitter down instead of him saying he pulled the splitter straight back which I thought he means pulling the actual shifter straight back. Kinda makes more sense. After all he did say 1 outta every 4 goes smoothly.

I have an issue like that in mine. 18 speed.

On mine once I've split to the high/low to get into the low side of 5th, I then split to the high side of 5th (Throw splitter forward, lift and ease back on pedal) which goes smoothly enough. I then pull the splitter back for the low side but before I can transition into 6th gear it pops and drops back to the low side before I can row the stick. This is the only gear it does this. I have acclimated myself to double clutching the high 5th to low 6th transition in order to keep what little hair I have left on my head.

This is behind a C15 @ 625hp CAT in a 2008 KW T660 pulling a 53' reefer that is typically loaded with 42k - 45k of dairy products. I dont know the gear ratio in the drives off hand,.. (Truck is in the shop getting new turbos and I dont have access to look atm) There is no reason or need for me to ever tach it beyond 1400/1500. I typically shift at 1350/1400. Drops down to 1150/1200 and that is the RPM range I use from coast to coast. I also average 6.35 mpg using this method. My overall average for the truck is 6.29 mpg according to the data logger. Taching to 1550 and above does prevent it from dropping to the low side before I shift into 6th. Anything less and I cant row the gear fast enough before it pops. On this motor,.. anything over 1450 is wasted fuel, so I dont want to make it a habit of shifting that high. Plus like I said,.. I've already acclimated myself to double clutching that gear. Just habitual now. Dont know what I would do if I ever get thrown back into a 10spd.

If the OP is in a 13 or 18 spd, I would try double clutching the problematic gear/s.

Ask the mechanics to put the diagnostic computer on it to see if any messages come up for the tranny. Sometimes it can be fixed through their computer system straight to the engine. The more advanced these trucks get the harder it is to fix em.