Subform - disable 'add entry' if mandatory fields not completed

Subform - disable 'add entry' if mandatory fields not completed

Hi all, 

I've been playing around with a few different ways to solve this and none of them work perfectly. 

Is there are way that I can disable 'add entry' on a subform if some mandatory fields in the previous row have not been completed? 

I.E. in row 1 the mandatory field has not been completed therefore they cannot proceed to add a new row 2 etc etc. 

Thanks! 
Lindsay