Doubts, uncertainty, things the prospect does not like. All this can, but need not, manifest itself in the form of an objection to your offer on the part of the client. Many salespeople fear customers' objections but really they should embrace them:
the fact prospects do not keep doubts to themselves shows they are interested in the product and are willing to keep talking about it. But you must know how to handle these objections so as to be able to close the deal successfully.