Bernotas05

I have recently put a 2800 stall and rebuilt the tranny on my 99 formula. The car was tuned but I have an issue with the car not shifting into third under normal driving. It will shift normal from first so second then once it shifts to third it feels like it is slipping and will just rev. But I tryed hitting it WOT from a stop and it will shift all the way through the gears without any issue while on the gas hard. What would make this happen and how could I get the car to grap in 3rd at lower rpm's.