The 25% member discount is available for rooms that come from the member's inventory. The first priority is to make point reservations- once MS is reasonably certain the member's inventory will exceed the demand for point reservations, they will make the discounted rate available to members. Even non-discounted cash reservations won't be made until MS is satisfied
that the rooms won't be needed for point reservations.
You should not have to call continuously as long as you have a cash reservation made thru MS. There is no "discount" waitlist however, so if you don't have a ressie made for "member's cash" you won't get the discount without calling.
We had made a July reservation at HH for a Sunday-Thurs point ressie and asked about a cash ressie for the weekend. At 11 months they wouldn't allow a cash ressie thru MS and suggested I call back in a few months. I called back at 6 months prior and reserved Fri/Sat with a cash reservation (still no discount available). In early June, about a month prior to arrival, I received a new confirmation in the mail for the cash nights- with the 25% discount applied. I had not called back to check, the discount was just applied when it became available (I was prepared to pay the rack rate for the weekend).
MS has also reportedly been willing to apply other discounts when they were lower than the 25% discount, so if you are aware of lower rate at a DVC resort, it can't hurt to see if MS will nmatch that rate.
Good Luck!