Rear Derailleur Issue

[daddyshambles]

Hi any suggestions please, the rear derailleur (shimano 6 speed) is indexing from the small diameter cogs up to the largest diameter cog very smoothly but will not move back down using the grip shifter. New inner and outer cables have been fitted and the inner is sliding in the outers very easily and the H and L limits are spot on. It does however move up and down using my hand on the derailleur. Thanks in advance

 

[MrJamie]

Do you mean it wont move down at all? So you can shift up to the largest sprocket from the shifter and the derailleur will just stay there even if you shift all the way to the smallest. Or just reluctant to downshift reliably?
If it wont shift down at all and just stays seated in the big sprocket but you can move it to smaller sprockets manually, id guess that either the cable isnt moving freely enough everywhere or the derailleur springy bit (very technical i know) isnt springy enough to pull the cable through anymore.
Just some thoughts, while you wait for an expert answer

 

[Rickshaw Phil]

My first thoughts are:
Are the new cables routed correctly?
Is the cable snagging anywhere?

The last time I encountered this problem it was caused by me fitting a light and accidentally clamping the cable down with it.

 

[gwhite]

This can be caused by too much cable tension which is overcoming the return spring tension (assuming a top normal der.)