SInger buttonholer for a 60's slant needle..thread...

Question

The machine works great, the bottonholer works fine but...occasionally the spool thread globs underneath and can jam up on the bobbin. What might I be doing wrong? I straight sew with the machine successfully but can't do 5 holes in a row without this happening ever so often. I am not sure if I am suppose to have the trailing top or bobbin thread in a certain place? Thanks I have 11 holes to do and have never used this before. Thanks, Laurie