As a former Petco employee, these are the things I can think of:
1. Mites! There is a 90% chance the snake has mites. We got our reptiles from CalZoo (I don't know where yours are from), where most of the reptiles are wild-caught and infested with both internal and external parasites. Every week I was having to being loads of reptiles to the vet just so they would be healthy enough to even consider putting up for sale. I would scrub the cages with mite chemicals at least once a week, but it was a futile gesture because the next shipment of reptiles would be infested with them, and I would be back at square one. I can't believe I didn't once manage to transfer the mites to my collection at home.
2. Check the heat! Petco seems to think that the lower the temperatures they keep their animals at, the less they'll eat and therefore the cheaper they will be to maintain. Get a infrared temp gun and check the temperature over that dinky little heat pad they have on the bottom of the tank. At the store where I worked, it was only a couple degrees above room temperature instead of the ~95 degrees it should be for sand boas. A cold sand boa will not eat.
3. Is the snake in a busy area? I've found that Saharan Sand Boas are very easily disturbed compared to Kenyans, and if the snake is constantly having people walk by and little kids banging their fists on the glass, it may be too terrified to eat. That combined with the fact that Petco seems to have some sort of policy that makes it against the rules to put more than like half an inch of substrate in the tank (I guess to stop the animals from hiding in it) means that the snake might not feel safe enough in the tank. Having shelters in the tank probably isn't going to be enough, it needs a deep substrate. I don't put any hides in with my sand boas, just a substrate that's a least a few inches in depth and a few flat objects for them to burrow under (they seem to like the pressure of something on top of them).
I really hope this helps. I still get really frustrated when I think of all the reptiles that are getting sick and dying at Petco just because the store wants to save a few pennies by not providing enough heat, underfeeding, not giving UV to the lizards, etc. Whenever the higher ups would come visit the store I would flip my b**** switch and let them know that this was wrong, but they never showed anything but apathy and just made up lame excuses. Funny thing is that whenever I tried to quit they would be practically begging me to stay, LOL. Hope this helps some, and good luck with the snake. 
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- Natalie
(San Francisco Bay Area)
