my truck has stopped advancing to a higher gear it will rev rpm on 1991 GMC 1500 Pickup

and advance faster but not making the shift. is there any thing like timing or vacume advance to check out before looking at the torque converter. there is no leaking of transmission fluid any where.

Asked by for the 1991 GMC 1500 Pickup
have you repl fuel filter lately? if plugged it will cause a trans like systom. also ck for plugged exh poss catt