MEXEM adds House margins to the Initial margin requirement for a contract. If you hold a position that has a higher Initial margin than Maintenance margin, it is possible for your Available Funds to be negative while your Excess Liquidity is positive.

Buying Power is projected as the current Available Funds (Equity with Loan Value less Initial Margin) times a multiple, so when your Available Funds is negative you will not be allowed to enter a margin increasing order.
Your actual Buying Power depends on your account type and the products you are trading and currently hold in the account.

Please be sure to visit the MEXEM Knowledge Base for more information on Determining Buying Power.