You might make sure that its not running extra rich.If it is very very rich the engine will not get up enough rpms to shift the tranny.Its easy to tell if its rich because it will be sluggish and not go very fast.It can be very rich and run good but not well enought o make it shift.See if you have a ton of blue smoke or even a little fuel running out of the exhaust.If either of these things are happening its too rich.If its very rich lean it out a little and see if it helps.You may have it tuned good and the problem is somewhere else but I always like to try the simple stuff first.Good luck.