Gretchen has been trying to teach glen, a preschooler, to pull up his pants by himself after using the toilet. she has been using a shaping model. she began by reinforcing the behavior with an animal cracker for bending over and touching his waistband. he is now doing that consistently. next, she wants him to start pulling his pants a little bit. however, she has become very frustrated because glen continues to simply touch his pants rather than pull a bit. what should gretchen do