Make sure the wiring is correct, if computer is unable to access the shift solenoids it goes into a default mode. If it is unable to access them all it goes in high gear.
Does the trans seem to be in high gear?
Determine what happens when the gear shift lever is used to force the trans to shift into 1st, then 2nd, etc and bypass any electric shifting.
If you have an ODBII code reader the transmission has its own troubleshooting system and might have trouble codes stored.