transmission problems on 2005 Buick Rendezvous

transmisson shifts hard . sometimes doesnt seem to matter hot or cold sometimes 1st to sec. sometimes all gears transmission shop code reader shows nothing and auto dealership shows nothing

3 answers
I had the transmission fluid changed and it has been good for about a year. It was slipping a little when starting off cold and that also improved but has started again.
SOME PEOPLE SAY USE LUCAS TRANSMISSION TREATMENT so I will try it.
my 2003 did this and i changed transmission filter and fluid and its running much better
How is the fluid level? When was the last time you did a transmission service?