So i've noticed that myself and others I know who are into making music all go through periods where you are less productive than at other times. For me this usually has to do with my state of mind, and when i'm in a good place mentally I tend to be productive and creative, easily finishing tracks and generally accomplishing a lot. When I have issues that might be affecting my personal life, I find it much harder to push through and complete a track. Making music becomes much more of an effort than enjoyable.
Everyone's creative cycle lasts for a different periods and is related to different things. For example, one of my friends is almost always working on something, he might have a week or two where he doesn't but he pushes through those less productive periods and continues to make music, although the quality varies depending on which stage in his creative cycle he is at. Obviously his mental state doesn't affect his creative output as much as mine does. I am more like what the producer Tarekith describes here, meaning I usually go through two or three periods a year where I have decreased productivity, with these typically lasting one or two months.
It can be a frustrating thing at the time, watching others get attention and putting out material when you cant seem to get started on anything and nothing works for you. But remember everyone goes through these periods and its important not to let it bother you. Let your creativity recharge from time to time and know that your next creative cycle will begin again shortly. Use the time in between these periods to do other menial tasks that you might not want to do when you are on a creative roll, such as:
Program Sampler Banks
Reply to / Send Emails
Increase Your Internet Presence
Organize Your Studio
Clean and Defragment Your Studio PC
Back Up Files
Dissect Old Projects
Listen to New Music
Doing these sort of things will keep you feeling motivated. Of course it's easier said than done and sometimes it can be nice to spend a weekend simply unwinding and maybe looking for inspiration somewhere or doing something else, whether creative or not. If you do this and start working on small things here and there, those periods of decreased productivity will be shorter since you are doing something, anything. Staying motivated and being dedicated is the key.