i am all with jerry on this one. try a piece of normal material. it sews ?
then shove in one webbing. it should still sew just fine. add 2nd webbing, and it might slow down, or you need to manually help the first stictch, but it should do it. more than that, you have to try it.
also, as was said up in the posts, if you pull the material things get bad quick. pulling unaligns the needle, and if you have ever actually Looked Close at how these machines work, thee will see that not much un-aligning is needed to muck up the results.
also i can confirm that bobbin tension is a very toutchy adjustment. if not actual backwards turning the adj screw on some units. watch the utube vids on it.
different color threads.. etc. that's all good council you are getting on your question.
my old singer 223 was boggy last week on thick material. it wanted to be opened up, and some oil drooled about inside, as well as a good wiping off so all nice and shiney clean. these machines make about 1/4000 of a hp, so any friction you can remove by thinning out dried oil goes directly into the material sewed. even too tight a belt will bog things down.
once it's all re-oiled, i wipe it thoroughly with a towel and windex. your machine wants to be clean and perfect inside to work properly.
note to the unaware : oil will dry out as arromatics leave it over time. this leaves us with a gooy thick ooze. more oil thins the ooze, removes some of it, and generally makes things better for mankind. it's not like a sewing machine consumes oil, it just wants new lube at occasional long intervals.
you can use an aerosol like TriFlon to good effect. although it may be too smelly for the delicate nose.
and needles ... have spares and be not cheap to deploy them when working thru a problem.