I have had similar problems on two bikes
1 2015
Ribble with 105 - just went "saggy" - apparently not uncommon around then.
2 2015 Vitus with Ultegra Di2 - wore out and also went a bit "sticky"
In both cases, peel back the hoods and find the code. something like 5700/5858/6800 etc. Agreed SJS best place to look.
To fit takes a bit of confidence. I used liberal amounts of isopropyl alcohol which helped a lot. Once on, fit like a glove