What I do is to ask at the LFS how much time they are there... (At least two shops in my city label the tanks with the arrival date). And I only buy the ones that have been there at least 10 days. There is a thread somewhere here explaining that bacteria on their stomach can be severely damaged after some weeks traveling from their origin. Thus buying a fish that has been at the LFS some days means that this fish has not suffered or is recovered... Before doing this I lost 2 of 5 fish, in just 24 hours with a very similar behaviour you describe. Once I did this, I could replace the 2 dead ones and an additional one without any loss.
Jordi
Sorry just started reading this journal as I'm definitely due another DW tank, but I'm aware this post was from years ago. Even so I thought I'd relay some excellent advice I was given on otocinclus, which is:
They were eating in the wild before capture, so there's a strong argument to get your hands on them the second they arrive in the store. Don't wait to see if they survive the LFS's care unless you're very confident the LFS is going to take exceptional care of them. This would include always putting new otocinclus deliveries into different tanks that have really decent algae growth, where so many LFS's have an appointed otocinclus tank that's permanently stripped bare of any algae, so they starve as they need a constant food source to graze on! Its actually a really good question to ask to assess a store's basic fish-keeping skills as to whether they rotate the otocinclus tanks?
Second is de-worming - few stores are willing to spend money on appropriate deworming for wild caught fish as its just cheaper to see which ones pull through than spend money on meds. Either way, its really worth using something like API's general cure for internal parasites the moment you get them home. This is just going to maximise the percentage of the food they take onboard that can go directly into their growth rather than growing more worms or parasites.
Third is whether or not the store is making some effort to wean them onto an appropriate prepared food, whether thats being able to see them munch on veggies or repashy or spirulina tablets - doesn't matter which that much, but the store making the effort is!
Lastly, if you have space, and they don't take up very much, buy at least ten otocinclus - they are very prone to stress related problems simply from being suddenly deprived of enough tankmates to shoal with, living with hundreds if not thousands of their kind in the wild. This stress combined with internal parasites will see them off, when in the wild they could cope with the parasite burden no problem.
Bottom line is that few LFS's know how to take really good care of otocinclus, or someone in head office has determined that its not cost effective to do so, so get them into YOUR care as soon as possible after they arrive. Of course if your LFS is good then giving them time to recover and to be fattened up make loads of sense.
cheers