Unable to Buy from cash balance - Schwab 529

Starting recently, I am unable to use my account's cash balance to record Buy transactions in my Schwab 529 accounts. I have always done this manually, but some time during December, it stopped working. When I try to manually enter the Buy transaction, it looks correct at first, but as soon as I select the security from the dropdown, the "Use cash for this transaction" box disappears. The cash value on my transaction then saves as "N/A", and the cash is not deducted from the account balance.