The issue with PPI meds and Sucralfate is that PPIs bind to proteins, called receptors, in the cell wall of the acid producing stomach cells, stopping the cells from being stimulated to make acid. They are very effective, but only if they can bind to those molecules. Sucralfate coats the stomach, which can prevent the PPI from contacting the cell wall, and therefore from binding the receptor molecules, decreasing the efficacy of the medication. Because of this, the PPI works better if taken well before the sucralfate. The issue of food and sucralfate is less clear cut. Eating food stimulates acid production, so having the stomach lining coated with sucralfate may be beneficial; however, if taken with food the sucralfate will be mechanically blocked from adhering to the gastric lining.
Just to make the issue muddier, PPI work better if taken with or just after food, since more receptors will be activated and, hence, available to be turned off by the PPI.