This is a well known limitation for FileMaker Finds. The most common solution is to evaluate the comparison in a calculation field and then search for a value in the calculation field.
Set up a calculation fields with FollowDate > LastCall as the expression and number as the result type. If you perform a find for 1 in this calculation field you will find all records where FollowDate > LastCall.
An alternative approach is to set up a filtered portal where PortalTable::FollowDate > PortalTable::LastCall is the portal filter.
Thanks Phil. That was more or less the solution I ended up deploying. I appreciate the response.