Are you sure they're L333? Are they LDA33? If so, they're baryancistrus, notoriously difficult to acclimatise after shipping - they (and some other sensitive plec species) lose their gut flora due to not feeding for weeks during transit, and then when fed again in their new homes it can take a while for that gut flora to grow back and process their food, and they can effectively starve to death before it does - even if they eat.
I'm afraid to say, when they're that hollow in their bellies, it is highly unlikely they'll recover
In future, it's best to only get these kinds of plecs when they've been in the LFS for a few weeks minimum after arriving from the supplier, and are eating well and not underweight (hollow tum, sunken eyes - once they have these it's usually too late). I found out the hard way with my gold nugget, and TBH he wasn't as thin as yours are
I agree heard this before.
Is that or internal parasites.