Reverse gear problems

It is very difficult to engage reverse.
When I attempt to do so the gear lever will nearly always refuse to "click" into position.
If I am fortunate to be parked on a slope with the car facing upwards and I let the car drift gently backwards it will then engage.
Also, if I stop the engine and then try to select reverse, it will sometimes go into position. I can then start the engine and go backwards.
Any suggestions please?