SS retirement checks are timed to arrive or direct deposit on the third of each month. SSDI depends on what week in the month the birthday falls.
You might want to contact her postal carrier to find out if other checks were delivered on time. If you call the 800 number for USPS and tell them you want to speak to the postmaster of her particular post office, they will give you the direct number (if it's not listed in the phone book).
Good luck. Hope it turns up soon.