Whenever something unexpected like that happens in BiaB, the first thing I do is shut down all programs and reboot the computer. Sometimes resources get mangled.

If the reboot doesn't solve the particular problem with BiaB, the next thing I try is Options -> Return to Factory Settings, which solves such problems about 99% of the time.

Study the situation in the Return to Factory Settings, it is a good idea to grab a snapshot of your settings while the program is still behaving so that you can make it revert back to your Custom Settings when you need to do this, rather than the Factory Settings.

--Mac