What kind of timescale do you have, and is there any way of tweaking your research design a bit, either to make it easier for people to take part or to make better use of smaller numbers? I've had recruitment issues with my PhD, but I can appreciate that time will be a lot tighter with a Masters. In my case I've made a few changes to my design so that people could participate in more flexible ways, and I'm also planning to give a reflective account of the recruitment difficulties. Do you have a supervisor you can talk to about possible ways around the problem?