Bike gears not shifting how they should So I've not cycled in a while but my step dad was using my bike and he has a habbit of breaking stuff. I have since decided to start cycling to work again and I replaced various parts due to rust etc (Chain, Cassette, brake pads, cables) and seeing as they didn't work properly in the first place I decided to take some time to tune the gears. Now I think I understand them but the issue I have is: On the cassette at the back the gears will happily and smoothly shift from 7 to 1 under load or not under load. But going from 1-7 it will ignore me when I switch to 2 but when I then go 2-3 it will skip over 2 and onto 3.

So the question is: Why does it work perfectly from 7-1 under any load but then under any load it will not go 1-2 but every other gear works? It's literally that 1 down to 2 that won't work.

Please take note that there is no grinding noise or anything when i flick from 1-2 it quite literally just ignores me and makes 100% no attempt to switch but it happily jumps over 2 to 3 when I flick down to 3.
A rear derailleur moves two ways: the tension from the shifter cable will pull it one way (usually towards the larger rear cogs) and spring tension within the derailleur will pull it back when you shift down into the smaller cogs.

It sounds like the spring tension is not strong enough to overcome the cable tension / resistance when you're shifting.

If you had to replace other rusty parts, you can try oiling or just replacing the derailleur cable and housing.  If those are rusty or sticky, then your derailleur spring has to pull a lot harder to get things to move.

I have also had shifting troubles when the derailleur hanger is bent, causing the mechanism to swing the wrong way when I was shifting.  You can get a bike shop to check / straighten the hanger, or if you don't have a hanger (claw type derailleur) you can try to do it yourself.