This is an excellent question and one that baffles therapists and patients to this day.
There are some days in which people report very little difficulty with an obsession or rarely perform a compulsion and other days in which the obsession is omnipresent or the compulsion is performed nearly nonstop.
Some possible explanations for this phenomenon follow:
• Other events or thoughts are distracting from the obsession that day.
• They recently gave in to the obsession or compulsion and exhausted themselves performing the ritual, which relieved them of it for a few days.
• General stress levels are known to have an impact on OCD behaviors, leading often to increased obsessions and compulsions.
• They may have missed a dose of medication, and the resulting change in medication levels had a negative effect on their OCD.
• They saw something on television or read a story about something that they fear, and it scared them enough that they went back to doing rituals.
These are just some possible reasons why people with OCD may see changes in their behavior. No one knows of an exact reason why it may happen, and it may be a combination of several of these or other reasons.