i see this sometimes. in fact i had a gig last night and had this problem. thankfully i have 2 w/ and the other looped just fine with a quick swapping of patch cables.
it does seem to happen more consistently on one of mine than the other. i wonder if it has something to do with placement of cues at the beginning of a tape, because that’s where i encounter the most weird behavior.
Another frequent glitch I have related to this theory is rewinding to the start of the tape and finding the play button doesn’t work until i jump or fast forward past the first cue point.