Some people are great with dating until things turn serious. Once it becomes clear that things could go all the way with this person, a range of anxieties can kick in. Some people have a terrible fear of commitment, whilst others become obsessive once they feel attached to a person, and the list goes on. If this is happening to you, be aware that there could be a reason that you are not conscious of.
Before you rush to question the relationship, consider whether this is not the product of an internal struggle that is all to do with you and nothing to do with them. The reason your excitement is not building up may be because you are with the wrong person, but it may just as well be because of your own inner resistances.