Some possibilities are:
Bullying and harassment
Financial or other worries
Drug or alcohol problems
Lack of training in the new system
Suppose your team member reveals that s/he is having to look after her/his mother who has had an operation and now needs a lot of support at home. You will, of course, be sympathetic and will try to find ways to help your team member to cope. However, you must also bear in mind that while these problems explain and to some extent excuse the poor performance, this does not mean that the level of performance can be allowed to continue. What might be the appropriate course of action in a case of this type?