Make sure the model isn’t empty (the code kind of makes it look like it might be)
There is a limit of 31 highlights, and additional highlights just aren’t shown. (Note disabled highlights also count towards the limit, so make sure to destroy unused highlights instead of disabling them.)
It looks like this is on Roblox’s end: when a Highlight is in a model and new parts are added, the highlight doesn’t update for the new parts.
You could try removing the highlight and adding a new one each time a new part is added (might work); adding a highlight to each part you want to select (will work); or creating a new model, adding all the parts to the model, then adding the highlight (might work).
Some workarounds from the thread above are adding a humanoid to the model briefly (not sure if I’d recommend that because humanoids are weird and can cause visual problems).
Setting the highlight adornee to the model you want all the parts to get highlighted, and then every time a part gets added, reset the highlight’s parent, and make sure it isn’t the same as the parts you’re trying to highlight. Hope this helps!