2001 Dodge Ram 1st to 2nd

i have a 2001 dodge ram 2500 truck wiyh a 5.9 diesel with an automatic transmission. I just changed the fluid and filter a week ago. now when the tranny warms up and i come to a stop it just stays in 2nd unless i put it into 1st on my own. it will downshift if i get on the throtle from the start but then i'm betting on it to much. i had a guy at a dealer say i need to flush the tranny but i think its something different.