If the following bar has a time signature change (or special barline?), make sure to activate insert mode before you create the time signature, so that Dorico adds the necessary time (this only works with addition of time, not subtraction)
Peter, does the measure show a 12/8 time signature? If so, it is a 12/8 measure. As a simple test, are you able to enter 12 eighth notes in the measure? You may need to go to engraving options to set eighth note beaming (grouping) as you would like. I hope I am understanding you correctly.