Our kennel rescue dog black lab hound dog mix had a terrible mental block around the steps. We found the sound of the motorized action of the stairs to freak her out.
Try locking the stairs open using the switch long before you approach. We also found placing her leash / leader on her (this action often places her brian in follow -the -master mode) helps too.
I think the open rear of the stairs also had something to do with her apprehension. We always placed her favorite treat at the top of the stairway (greenie bone) for her to associate the completion of the climb with goodness. But it was not placed in sight or dangled as a carrot. It was a discovered treat.
Eventually she climbed up with no issue. But the longer between trips the more often we have to retrain her. So at home, when I do work inside I offer bring her in with me for "practice". Note that I had an issue with my Lippert stairs within 2000 miles and after finding the appropriate person at warranty service we replaced the 3 step with a more reliable 2 step Kwik step. When we need to we supplement the third step we use a folding aluminum step from CW that doubles as a foot rest on the passenger side.