The Republican nomination race has three more stages to go, and each of the three candidates left in it is concentrating on a different one.
First, the candidates have to win votes in the remaining contests to maximize the number of delegates bound to them on the first ballot. Second, they have to prevail in selecting delegates: making sure that their bound delegates stay with them on subsequent ballots should any occur, and that unbound delegates side with them. Third, they need to get delegates who are not with them to like them, so that they might vote for them on later ballots.
Donald Trump has so far put all his effort into the first step. In part that's because he has been unwilling to spend money to build an organization, which he would have needed to compete in stages two and three. But while he has done well in the primaries, he has not done well enough to dispense with wrangling delegates. It is now very late for him to begin.