I've found in my own workouts that there is a relationship between momentum, motivation and competition. One thing nice is that when motivation and competition fail, momentum can still carry the day and the week.
If the standard practice is way above average, then a bad day or week can result in what others would consider an accomplishment. That is, if the average day is working out for two hours plus, then achieving merely one hour per day, while definitely sub-par, is still considered very good (by others with less rigorous schedules).
This is how momentum can help when motivation fails, or there is a problem with injury or scheduling.
This can come and go, so it is best to treat it as a cyclical phenomenon. That is, don't depend on it, take advantage when it shows itself, realize it will not last, and seek more motivation when it becomes scarce. This leads to a third element, competition.
I've found that the presence of another runner or cyclist brings out the competitor in me. Not every time, but very often. I ran a 10k on Sunday, April 14th and the presence of a few hundred other runners really pushed me to run about as fast as I could over the course of the entire 61 minutes. This time in turn jacked up my motivation to become faster (and run a sub 4 hour marathon in December).
And so, when there is a lack of motivation, let the momentum carry you, and seek out more competition.