In response to an e-mail inquiry that I sent to MyPoints about missing points, I got this statement which may explain why they are holding some point transfers.

"Please note that Points earned at participating sites are credited to
your account as soon as we receive confirmation of the transaction from
the vendor. This means that Points for making a purchase or registering
for an online service, may take up to 20 business days to post to your
account balance. Points then become available to spend 30 days later. "

Sounds to me that you must hold the points in your account 30 days before spending them.

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CO Platinum, USAir Chairmans,
Marriott Platinum, Priority
Club Platinum