Brompton derailleur gears sticking

[buckett]

The derailleur gears on my brompton aren't changing well. They will eventually switch both ways with repeated gear changing, but this isn't practical. The cable isn't sticking and the guide is reasonably free. Any ideas as to what to investigate?

Might the chain tensioner need replacing, or there spring on the cable?

Thanks

 

[alicat]

The last time that happened on mine a good clean fixed things....

 

[Globalti]

When was the last time you replaced the cable outer where it curves round to the rear derailleur? They wear fast there and need replacing regularly to maintain shifting performance.