Riding back yesterday bike started acting a bit strange, when pedaling it was absolutley fine – but when rolling the cogs at the back where shifting back and forth and the chain was sucking up into the frame. I took the wheel out ha a look couldnt see anything wrong but its still happening? is this an easy fix

Sticky freewheel? How much resistance is there when you back pedal by hand?

What wheel / bike / mechs?

Thought id replied oops..its fine when pedalling, going through the gears is fine its just when im rolling. I have a shimano deore cassete probably about 3-4months old same with chain. I run a narrow wide ring up front

freehub body like matt says, take cassette off and try and get the freehub off (11mm/12mm/pull on it) to take it off and check the bearings are not full of rust/no grease.

What Matt said, sticky freewheel!

Or worn/sticky jockey wheels?

Sticky freehub. I chased problems round and round my drivetrain about six months ago. Put an old mech back on, swapped jockey wheels, and other things all to no avail.

Eventually found that the bearings inside my hope freehub were bollocksed.

From the symptoms I would definitely put this as the cause. Even sticky jockey wheels only cause a problem when you back pedal ever so slightly.

Had similar.
Found some fishing line caught/wound between cassette and wheel.

Had to strip the whole thing… No hook luckily