annieg it is good that the feed dogs are now up in place. Now with regards to the stitch length etc. I have just been looking at your photo of the machine, and over on the right you have two places that are obvious to alter for something, which is which I do not know, but how about resetting them to 0 or to 1 and see what happens. Then you have another smaller dial or something quite close to the other two, and I presume these are for some sort of zigzag, again try resetting that back to 0 or to 1 and again see if there is any difference.
If the machine had previously been set for darning, the whole purpose of this mode is that you have your fabric in a hoop, lower the feed dogs and then you do the moving of the hoop to create the darn, thus the machine probably does just stitch in one place only and to get the coverage of a darn it is down to you to move the hoop. So just maybe the last time the machine was used was for darning and that is why the needle is just going up and down in the same place.
You have nothing to lose so try and see what moving the various switches do - don't interfere with the tension disc which is over on the left, leave that one well alone for the time being.
I had borrowed a machine last summer and my friend hadn't used this for months on end, but it had obviously been lent to someone else and the state it came to me in was disgusting, it was dirty, it had been set for some automatic patterning, a very thick needle had been left in place, and various other settings had been altered. Why when a machine has been used for something rather than straight stitching, is it not put back to neutral as it were ready for use. As it is my friend is not wanting her machine back at the moment, and it just happens to be an identical one to my own Elna SU - all she has left over at her house are the instruction books. However as I know my friend rather well, it is just as well it was me who discovered the state of her machine - she would have got in a panic and not know why it wasn't working as she expected it to work. Oh yes the part of the wheel you loosen in order to wind a bobbin was jammed so tight in the end we had to force this by using a tall anchor embroidery wooden reel put under the needle retaining screw so that pressure could be used to free the inner wheel without the needle going up or down.
So please do not lose hope yet with this machine - it might just be something so simple only you do not know yet what it is stopping this from stitching.