This happens quite often with custom models, and it has to do with their extents. Open the model in Magos' Model Editor (see the Tools section) and go to Edit -> Calculate Extents. If you still experience the hp bar problem, then you may need to manually input extents (idk if Magos' editor allows you to do that).
Alternatively, if you are scaling your model in any way (e.g. object editor or triggers), you might want to consider scaling the model in Magos (Windows -> Model Editor, and then use the transformation tools with all vertices selected), calculating the extents, and then importing it again. Sometimes that can fix it.
If you need to manually set extents, then try Edit -> Model Properties. If that doesn't allow you to change them, then you may need to convert to .mdl, edit the top extent, and then convert back to mdx. Sometimes people will also just add a plane above the model's head or a rectangular prism to give the model the extents they need, and then they'll have Magos calculate the extents appropriately.
