Slide lubricant, bad alignment or not enough torque ?

[Robinwales]

Silicon spray can cause galling at a metal to metal sliding contact. It is famous for destroying air guns. While you clean it out you can investigate the problem.

Can you feel excessive drag when you wind the slide screws by hand?

Are you blessed with an oscilloscope? You could check out the Arduino step pulse. Is it excessively up or down? A square wave is perfect.

You can get small amounts of slideway oil, no need to get the whole 5 litres.

Unfortunately, don't have an oscilloscope.
When turned by hand not sure I recognise any excessive drag, although I have nothing to compare it with. I've disconnected the threaded rod and tried sliding the carriages by hand and there is some metal to metal suction to get them moving. It's not possible to push them with your fingers and for them to slide from one end to the other, if you know what I mean.
Will try some slideway oil.
Thanks