Cash Donations

My wife and I are filing a joint return.  Our AGI is roughly 75,000.  Our donations are roughly 40,000 in cash only.  One of our cash donations is for 24,000 (all by itself).  TurboTax is only allowing a total of roughly 12,000 in cash donations.  Why?  What formula is TurboTax using to disallow the rest of the donations?  What do I need to do?