Which program - Adagio Payables or Checks?
What is the version? Check from the Help / About menu.

Does the same problem happen for all check specs, including sample specs, or just the one you are working on? Perhaps there is a problem just with your particular spec file.

What is the Max Details value as set in the File / Specification Options. It should not be more than 100.

Previewing of check specs from the designer should be relatively instantaneous - 2 to 3 minutes is not normal.