I'm going out on a limb:
You've not stripped and greased the linkage in a wile and it's sticking. The gear lever moves down into 1st, Up into neutral and up into 2nd, but then it isn't returning cleanly to position, so isn't moving the shift star to the next notch.
To test put some oil / WD40 or similar around the gear lever where it's at the footpeg and wiggle it about a bit. If it gets easier then you've found the issue and can strip and clean it.
If all else fails bring it round to mine and we'll sort it, but you should only need a 10mm spanner and a 6mm hex
Jambo