What are some reasons followers of Christ hold on to issues of a person’s past?
In answering your question, the possibilities are far too numerous to contemplate. But there is one major contributor that many don't want to talk about, trust. Many people today believer and non believer alike, simply are not trust worthy.
In many cases where one believer offends another and the relationship is now limited and changed from what it was previously, many assume that forgiveness is still withheld. But many times this simply is not the case, many have a problem discerning the difference between unforgiving and distrusting. Once trust has been violated in a relationship it is very hard to restore and it is the responsibility of the offender to earn back that trust. One can be forgiven of an offense but still not trusted. There is no scripture that I can find where believers are commanded to trust one another, especially those who have proven themselves to be untrustworthy.
Dunno, Apostle Paul remembered the disloyalty of some... (2Ti 4:10) For Demas hath forsaken me, having loved this present world, and is departed unto Thessalonica; Crescens to Galatia, Titus unto Dalmatia.
Great examples of my point, Mark is also a good example. There is no evidence in the scriptures to suggest Paul was being unforgiving towards these individuals but he sure didn't trust them, Mark eventually earned back Paul's trust but it took some time and energy on Mark's part to regain it.