[60041]

The indicator must go to green when the bar is correctly engaged: if it stays red, there is a problem.
It could be something as simple as a bit of grit or rust in the socket, so try to clean it out with a bottle brush or similar; likewise make sure the end of the bar is clean and the locking plunger is not stuck. I sometimes give my socket a blast of compressed air and also use a dry film lubricant on both the bar and socket. Wet lubricants like WD or oil attract the dirt.
The lock has no bearing on whether the bar is secure or not, all it does is prevent the release knob being turned; the locking mechanism for the bar is entirely separate.
When you remove the bar, you turn the release knob until it clicks, the locking plunger will be withdrawn and the indicator will be red; it should stay in that position until you come to re-fit it, when pushing the bar into the socket and giving a sharp push down should result in a sharp click and the indicator turning green.
There has been the odd occasion with mine where I have removed the tow bar carelessly and the release knob has not clicked fully into the release position, which has meant that the locking plunger has not locked back into the end of the bar and this makes refitting the tow bar impossible until the knob is turned and the plunger locked back properly.