I have an older MTB with Deore shifters, & when shifting the front the derail won't stay in position; ie, if I had a super thumb & could just keep it on the shifter it would stay on a certain chainring; course, this is not an option.
Being really cheap, I tried taking a shifter off a discount store bike that was given to my kid, & it won't work either (it says it's STI). I can shift from granny to middle ring, but after a little bit it just clicks itself back down into granny.
Don't really think I have a derail. problem, but didn't know if I had the cable too tight, or was just being too cheap.