Force Stemlet Beam

This is something not relevant to a single project but happens every time I want to use it. If I have, for example, an 8th note triplet with a rest on beat 1 and I select the whole thing including the rest and then in beaming select force stemlet beam, nothing happens. When I repeat the process I get the stemlet the second time, but why not the first?

Hmmm. Thanks for the quick response dankreider