The reinforcers aren't reinforcing:
 -they are not desired by the student
 -the ratio of task/demand to reward is too high
 -"satiation" has occurred (the student become "bored" with the reward)
      +given doughnut holes/pieces every 5 minutes
      +comic book to read (again, for the 12th time)
It is prudent to have a "smorgasbord" of rewards so that the student can select a reinforcer from the inventory that is of interest at that moment (i.e., Trading in points or tickets for items and privileges of various values).
 

Inconsistent delivery of reinforcers
 -assistant forgot to administer them
 -teacher failed to administer them due to busy schedule
You haven't kept up your end of the bargain, so why would the student do so?  You have been unpredictable and untrustworthy.  You must be consistent in reinforcing appropriate behavior.

The student is unable to engage in the alternative/desired behavior
 -not developmentally ready/physically able
 -the behavior is too complex or advanced for the student's cognition or ability level
 

The reinforcers were faded out too quickly
 Avoid going "beyond the data".  Avoid increasing the work-to-reward ratio too quickly.  Use your continually collected data to figure the correct time interval.  Don't lower the DRL limit too quickly.
 

The reinforcers were faded out too slowly
 Some youngsters desire a challenge to their abilities.  Failure to the adjust schedule to match the data has created a lack of contest.  This problem can occur when working with two youngsters at the same time, using the shorter of the two DRO time intervals.  The student who could handle a longer time interval does not feel challenged.
 

"Generalization" to other environments/conditions was not promoted
    It is important to help the youngster display the appropriate behavior under changing circumstances (e.g., staff member, time of day, classroom) so that the correct behavior is displayed outside of a certain time and place.