Sentry animations are excessively annoying to tamper with, and require a certain set-up to work properly
If you just decompiled the default model and just recompiled it using the QC the decompiler spat out, it's going to be broken
The decompiler can't recognize certain values and settings from the compiled model, so when it decompiles, it either guesses or just says it can't find the correct value. This is a big issue with layered animations, which the sentry uses for pretty much all of its animations.
If you're just trying to replace the model, but keep the skeleton and animations the same as the default one, there's a relatively easy fix
If you're trying to replace the skeleton or animations, the fix is much more complicated, and requires a lot of tinkering.