To avoid clubs using more Credits than they own, Virtual Manager has a security mechanism that sometimes locks certain Credits. In this way clubs will always afford to complete a purchase.
The mechanism locks the sum of your bid's auto bids that has not yet been accepted plus the sum of the auto bids of those auctions of which you are the high bidder.
Sometimes it can be wise to cancel old bids to free funds for new bids. This can be done via Trade > Bids > Outgoing bids.