I think there are two problems here. The first is that the gear changes should be made while taking pressure off the pedals or by backpedalling. This is how it was with Sturmey Archer gears. There is no need to maintain pressure because the gears are spring-loaded to change up or down.
With the second problem, changing into 5th gear, I had the same with changing to 4th. I checked that the two yellow marks lined up and then took up the slack in the cable by adjusting at the top, little by little.
However, I found that the problem recurred if I left my hand on the gear-change after the "click". What I think happens is that if pressure kept maintained on the control, then it is transferred to the cable and the gear does not engage properly.
Since keeping all this in mind, it workes properly now.