Can't Seem to Make Client Side Validation Work on a Specific Form

I have two very similar forms and they both have client side validation rules set on various inputs.

One form’s validation works perfectly okay, and the other form’s validation does not kick in when the form is submitted.

Is there something obvious to look for to debug this?

The forms are really big, but am happy to paste parts of them here if anyone can tell me what to look for!

Thanks!
Antony.