Putting the "=" sign, as in your first example, tells VFP to treat the setting as an expression. So if "tablename" is a character variable, VFP will initialize the RecordSource to whatever value "tablename" holds. In your second example, VFP will look for a valid alias or whatever with a name of "tablename".
Since you indicate the first example works fine the first time, but not after you close and reopen the form, look to see whether the alias specified by "tablename" is still open. Also, check the value for "tablename", it may have changed.