For many of them, writing letters is pretty low on their priority scale. And try not to push your professors too hard to write letters. If one seems reluctant, it is probably better to leave it at that and to pursue other avenues. Some professors will insist on writing closed and sealed letters, and will ask you to waive your right to read them, but many will readily give you a copy. If you have reason to believe that the professor has a chance of writing something pretty negative about you, by all means, insist on an open letter or no letter at all.
Nothing can ruin an application as quickly as an outright negative letter of recommendation.
One student once wrote a letter of recommendation for himself and asked that the professor sign it and mail it. The professor did sign it, but right next to her signature she stated emphatically that she did not write the letter. But in general, professors tend to leave out the negative and will generally try to paint an honest picture of the applicant. A closed letter helps them do that. An open letter is a safer bet, but it may not allow them to be as honest as they would like to be.