There is a problem with credit card authorizations on my site.

There is a problem with credit card authorizations on my site. Some go through while others don't and they wind up in my incomplete orders. Any explanation for this? Anyone else having this problem?

Customers are complaining that when the click "place order" nothing happens.  I get the order as incomplete, but not paid.  Requires me to contact them and get payment offline.  Please help!