If you do a search you will see that the Vauxhall derived gearbox in the 20V is known for such selection issues. Not all have problems but many do (including mine). You may have a fault but its more likely that you have a notchy one. I tried lots of things (fresh bleed, oil, reducing oil level, selector linkages) but to no avail. Slave is inside the bell housing so tricky and expensive to replace in any case.
I find using more rpm with a ligter throttle helps - eg if I use 2,500 rpm the 1-2 change is much better. Putting medium pressure on the gate and letting it drop in when its ready helps a bit as well. If its any consolation you do get (mostly) used to it....