Sewing machine ok with test fabric, not with proper fabric

[BreakfastAtSquiffanys]

Basic electric machine, variable length and width of stitch so will do plain, zigzag, forward and back. Mostly used for soft furnishing.

I need to turn up a hem on overlong casual trousers. They are made of a thin rather slippy material.
If I run a remnant of random fabric through, it sews fine.
But the trouser hem.... the stiches don't appear. The top thread lies along surface.
I thought the teeth underneath might not be able to grip the fabric, so put a bit of tissue under fabric. no success- holes in fabric, no seam in hem.

So in desperation, I set to zigzag.

Which produces a straight seam!

What on earth is going on?

Thanks

No expert but it sounds like you need to adjust your tension, do you have any material similar to the trousers to practise changing the tension on? Think it has to be looser for silky materials but I could be wrong

I used some synthetic silk as my test fabric

It's just so weird that the zigzag has come out as a striaght seam.
I darent try the second hem yet!

If the zig zag is coming out as a straight stitch, could it be that you haven't set the stitch width?

If the tension was too tight the thread would just snap. Has the lever/switch been knocked half way between zigzag and lockstitch? One of my machines does this sometimes. It's as if it doesn't click off the previous selection. Also, check that it is all properly threaded for the bobbin and needle. Sometimes you just need to take it all out and start again. I think mine has gremlins sometimes as there appears to be no rhyme or reason as to why it is doing! I doubt that the fabric is too thin if it is trousers, more the machine settings/threading.

Yes, check stitch width setting as elephant says.

It's an very basic machine, the width setting is just a sliding knob.

The straight hem appears along the far right of zigzag setting

I suspect gremelins.
Or sheer wanton cussedness!

Have you also changed your needle to suit the new fabric? My machine is a git has had this problem with thin slippery fabric.

Sorry - mean to say to a different type of needle (ie specially for thinner fabrics)?

Hand sewing job.