Bit of background: I work in a team on an hourly rate in a busy logistics environment in the UK. We work 5 days a week on shifts either early or late. Frequently due to holiday or sickness we get asked to do overtime. Some people do more than others, usually because they are happy for the extra pay, but certain people never help out.
However, recently I have had enough. There are certain people I work with who don't pull their weight on a normal shift, either through incompetence or laziness. Several of us have brought this up to the manager who has promised to bring it up but never has. It has gotten to the point where I want to turn down overtime because I am fed up with coming in only to have to pick up other people's slack. I'd rather work understaffed shifts and do my bit and go home than work overtime and be dumped on.
My problem is I find it very hard to say no if I don't have a valid excuse, usually out of obligation and guilt that I am making it harder on those who actually do their bit. What are peoples opinions on this and how do you approach it?