Can sleep apnea get worse even using cpap machine

I was diagnosed with severe sleep apnea about three and a half years ago and use a cpap machine on set on 10. Recently I have started to get quiet tired again like I was before being diagnosed. Is it possible for my sleep apnea to get worse even though I use a machine? I also had breast cancer in 2008 and have completed 10 months of treatment - chemo and radiation and work about 25 hours a week