Isolating valve and flexi tap connector - poor connection

I am having difficulty getting a leak free connection between the BSP end of a flexi tap connector and a chrome isolating valve. The flexi has a rubber seal but what I have realised is that the isolating valve thread is not quite long enough to fully receive the flexi. When fully tightened there is still a slight in and out movemento of the flexi pipe.

I have unscrewed the BSP end of the flexi and connected it to a spare brass coupler which has a slightly longer thread, resultng in the fexi sealing correctly.

Is this a common problem?
Can I add, for example, a 2nd rubber seal to get round the problem?
Are there any alter alternatives other than replacing the isolating valve with a longer threaded straight coupler?

Many thanks.

yes the end is not designed for that connection, the fitting that seals has a larger surface to seal against

Thanks 'cross thread' for the very quick reply.

Please can you just clarify. Are you saying that the BSP end of a flexi tap tail is not designed to be attached to an compression isolating valve?

I'd rather say a compression fitting (minus nut and olive) is not designed to take the female end of the flexi hence www.toolstation.com/tap-tail-adaptors/p14081

That's really helpful, thanks polesapart, I will get some tomorrow.