Couples include married couples and unmarried couples. In my experience, the greatest roadblocks to resolving relationship issues are resentment and communication problems.
Dealing with communication problems can be fun or disturbing or enlightening.
Oftentimes, clients discover that what they have assumed about their significant other has been inaccurate. Sometimes clients uncover feelings that the other has been experiencing. Doing so can be pleasantly surprising or sometimes upsetting.
Whatever the case, improving communication is almost always necessary to resolve relationship snags.
As for resentment: how long has this resentment been building up? The longer it has stacked up on top of itself, the greater the patience needed to slowly dissolve this resentment. It will require each member of the couple to let go of something. What that is, will come out during the counseling sessions.