@CircuitD Sorry for your frustration. I have my own frustration with retail, but I also enjoy playing it. If you direct your Sim to do tasks they don't do them? I haven't had this issue, unless their needs are low or the queue is dropping over a small number of queued items or for another reason (there have been bugs with that in the past). Unfortunately, you may need to check that your employee has his/her task still assigned periodically. Also, the task assignment you made for them does not carry over between shifts. There are perks you can purchase that will shorten the time to restock and other positive/helpful aspects. I think the better your employee's skills are in relevant areas, how much you have trained/praised or corrected them, given them raises, if you have a positive relationship with them, if you don't keep them overtime, etc. can play into it, if memory serves. (Note: I have not played retail for several months, so I can't speak to any new issue.)
Personally, I tend to not depend on retail employees. I keep a couple of my household members on the lot (and even keep it open extended hours, sending home and bringing to the lot other household adults), and I pretty much micromanage through them. I have found that adding/upgrading with perks helps a lot. I do hope that retail will be one of the upcoming 'pack refresh' targets!