keyboard input focus

Having trouble with keyboard input focus, specifically button states, I think.

scenario: user activates button by tabbing to
it and hitting the enter key. Program disallows
this action under certain circumstances and pops up a message box indicating why. When user clicks on ok button of messagebox, program uses SetFocus to editbox with bad information. user
then corrects that info, tabs to button, but enter
key will NOT activate the button now, UNLESS user
tabs past the button, then backtabs to it. Mouse action on the button works just fine.

ANY IDEAS as to why the button won't activate off the return key would be greatly appreciated.