jack might have stumbled once or something & it spooked him or he saw a spider or a shoe or anything. could be a shadow once he didn't expect or a noise startled him...could really be anything..are the stairs open backed? stairs can be a tough one for some dogs. you can try leaving treats on the stairs to see if that will entice him to go up & down. don't force..make it a game & let him do it at his own pace.

i have a foster now who fell down some stairs at some point & he is very wary of my back stairs which are open backs. solid ones he is fine on but other ones...he almost refuses to use.

the separation anxiety is a hard one to get over for the test..i had a dog who would have been an excellent therapy dog but that was his downfall. he was fine with wheelchairs, crutches, leaving food, etc but the anxiety was something he could never master...

good luck tho...keep trying if he doesn't pass the class the first time..retake it.