Social loafing: occurs when an individual is less productive (exerts less effort) working in a group/team than s/he is working independently.
Most likely to occur in large teams where individual output is difficult to measure
When task is boring
When objective is not important
When team doesn't value membership
How you could reduce this is by
Creating some form of performance measurment for each individual, that way they will be motivated to do well, as it will be indicated to their peers and managers
Add some creativity to the task so that it is not boring, when something is fun people are more motivated to do it
Make the importance of the task known, explicitly state its impact and consequences
Increase team cohesion through team building activity (just a suggestion)