105 RD Not Dropping Down Cleanly

I was having the same shifting problem with my 105 5700 RD. Kept changing the inners and outers to no effect. Eventually after 20,000 km of dodgy poor timed shifting the internal spring gave up. I was outside a bike shop and bought a new 5701 RD, fitted it on the street and immediately could shift no problems. I guess the internal spring was the problem. New mech is crisp and makes the bike a lot more enjoyable to ride.

The 5700 105 setup is notoriously difficult to get right, across both rings, and all sprockets. I changed the 11-25 5700 105 cassette for an 11-28 6700 Ultegra cassette, and kept the FSA chainset. That did make a bigger than expected difference, and although I very rarely use the granny ring, it does seem to shift a lot more cleanly with the Ultegra cassette.

Well chaps I think this is a case of RTFM! Shimano tech docs are very clear about installing asymmetric chains. (I believe they are now phased out.)

I recently had the same problem on 5700, only happened last few outings, was previously slick as shimano can ever be, less than 1000miles on these levers and cables perfect inner and outer and rear, it was similar perhaps not the same issue but hanging on downshifts, and generally messing around.. i know everything was right as i built the machine myself, all from new kit, and then by accident i noticed that the right shifter wasn't returning to the vertical neutral position after a shift, would have probably noticed this sooner but i use on a winter bike a right front brake cross top lever, and rarely brake from the shifter, we are using country roads so not a lot of braking involved either.. the lever could be assisted back to neutral but it affected the way and the quality of the change, sent them back to the supplier under warranty[ 2 years ].. back with me inside 4 days, they say it was dirt and grease, i think its a weak spring, they tested it and said all good... not yet refitted so we'll see what happens over the next month.. warranty runs out june i think..

It could be the rear "loop" ie the section of housing between the RD and the frame is too short. This causes too much tension. It should be a generous semi circle and then some. Perhaps post a pic?

I had something similar on Ultegra. In the end my bike shop took off the rubber nipple on the end of the final outer leading to the RD as it was working itself inside out and stopping the cable running smoothly. No problems since but you need to put some grease on to stop water getting in.

Almost certain to be cable related. Try removing the inner then threading it back through from the lever, the amount of resistance you feel can be a good indicator of where the cable is sticky. The cable under the bar tape can have tight bends depending on the bar shape and how it's been routed so you may need to wind the tape back and replace that.

I've had this issue a few times on my bike, clean and oil (or replace) all the cables between shifter and derailleur.
Also check that there's nothing actually inside the shifter barrels-a bit of twig or grit, just to make there isn't something physically blocking the shift from happening.

How old is the chain and cassette? How old are the cables? Are the pivot points free and easy moving? Is the FD correctly aligned? And while RD alignment being out normally affects the lower gears (Bigger sprockets.) a quick check of that wouldn't go amiss.