Try manual mode, which isn’t true manual, just basically limits the upper gear.
Don’t need a trailer or a hill, but you can figure out what speeds you can run in what gear near max rpms.
You can’t just expect to mash the skinny pedal and have the truck anticipate what you want it to do. Next gear down might be at or too close to redline for the Ecm to allow the shift.
Now some newer trucks also have inclinometers in them and actually know if you’re going up or downhill and shift accordingly.
Near 0% chance there’s something wrong with the truck ifnit otherwise downshifts and upshifts fine.
At the expense of beating a dead horse, trying to squeeze every last rpm and hp out of the truck to get up a hill is why people recommend a truck with more torque. IE oil burner.