Which filler to use on stairs? Kitchens, doors, rails, stud, tables, chairs, stair cases, garden furniture etc... Find answers and ask questions here!

[itsaproject]

I am having a carpet runner fitted to my stairs. There were quite a few holes and splits in the wood which I filled sanded and painted. After a few weeks cracks have started to appear. I used a ready mixed wood filler. Can anyone suggest something flexible enough for the job?