Some people do not put in an equal amount leaving others with a heavier load. They may not be present at meetings, not complete work to the required standard, or fail to deliver work to an agreed schedule without allowable excuse (e.g. illness).
Social loafing should be addressed as soon as it is noticed either at a team meeting or through a one-on-one discussion. We would recommend that the reason for social loafing is solicited first as sometimes there are valid reasons for this sort of behaviour. It may be that more support or a different schedule is required for the person.
Graded assertiveness techniques can also work well here.