To avoid clubs using more Credits than they own, Virtual Manager has a security mechanism that locks certain Credits that are already in use. By doing this clubs will always afford to complete a purchase.
The mechanism locks the sum of your bid's / auto bids that have not yet been accepted, plus the sum of the auto bids of those auctions of which you are currently the high bidder.
Example:
You have placed two bids at 20.000 Credits and 30.000 Credits. Besides them you are already leading an auction with an auto bid of 50.000 Credits.
You have 200.000 Credits on your account.
This gives an available amount of: 200.000 - 20.000 - 30.000 - 50.000 = 100.000 Credits. You can therefore bid a maximum 100.000 Credits on any other player(s).
Sometimes it can be wise to cancel old bids to free funds for new bids. This can be done via Market > Buying > Bids.