you may have to remove the tranny and unjam the second gear cog .all it is ,is a small metal arm that flips out from centrifical force from the rpms andshifts it into second
there maybe something that came loose inside and jambed it ,there is a lot of different causes for it not to shift or shift way early ,
try turning the set screw lock wise till snug (do not over tighten)thne out 4 to 4 1/4 turns and see if it shifts or is not shifting too early
if it doesn't shiftand still seems doggythen ripper out and check it
btw:do not use any type of oil or grease inside the tmaxx tranny ,i always used powdered graphite in mine and everynow and then gave it a shot to keep things working good