If you qualify, it is backdated from the date of application.
Entitlement isn't just based on the financial calculation shown in the sticky post - there are other criteria which must be met (and which all have to be investigated and checked before a supplement begins payment)
- ability to pay the mortgage when the loan was taken out
- "reasonable interest" (a grey area, but as a guide, the interest paid may be compared with the rent limit for the area)
- arrears, if any
- purpose of mortgage - if any of the loan was used for something other than purchase or upkeep of the property, MIS isn't payable on that portion.
Might there be something outstanding from you or your lender that is holding up the application? If you haven't spoken to your CWO recently, it might be worth checking just in case.