When you can't seem to practice magick it points to unhealthy emotions, thoughts, energy/spirit, or physical well being.
If you can scratch out a few of those, you'll be able to narrow it down. Here are a few possibilities:
(A) Stressed and therefore scattered causing you to be incapable of concentrating correctly
(B) A mental block that is preventing you from 100% desiring to practice magick. This could be anything from worries, depression, or having too much to do and too little time to do it (feeling hurried).
(C) Lacking energy (physical/spiritual). Did you overdo yourself? Do you feel tired? Even less sleep can cause this. Did you experience something traumatic?
I'll be waiting on your response. Hopefully something I mentioned will click. You need to identify the problem before being able to correct it. =)