Lack of motivation, i.e. Absence of "burning need" to learn the stuff at hand. Also, inability to visualize the end-result/reward.
Trying to look at the task as whole than in smaller parts: i.e. looking at a 720 pages, mostly text-only O'reilly programming book.
Multitasking: Checking news feeds, twitter and trying to learn/code/read simultaneously.
Distractions: people around, location, short attention span.
Not able to get into the zone/flow: This usually happens when,
goals are not clear
feedback is not immediate
lack of balance between opportunity and capacity
http://en.wikipedia.org/wiki/Flow_(psychology)
Appropriate reward and feedback system is really important in learning.As per some studies, it relates to certain neurotransmitters in the brain i.e. dopamine.