I too have expierenced this problem. I was taking lorazepam and zopiclone (sonata) for insomnia and as they usually have very noticable effects, some nights I found them innefective. I was baffled, until one night when i started coughing heavily and regurgitated the intact pills, which apparently were lodged in the back of my throat.

If pills get lodged in your throat, try drinking something first, so your throat isnt dry when you swallow it.