Grass sticking on a mower deck is a very common problem. The root cause is the moisture in or on the grass which acts like a glue and joins all of the cut particles together. Because of this adhesive action, grass sticking to a mower deck can become a real nuisance. It can become so bad as to clog the discharge chute, leave huge clumps of grass on the ground or even stop the blades on a mulching mower. Unless you can reduce this type of grass sticking, you'll have to stop several times while you are mowing, turn off the mower and clean it out manually.
Cut your grass when atmospheric wetness has evaporated. Rainy days are never good days to cut the grass, nor are mornings of heavy dew. On these occasions, wait for the moisture to dry off, which ideally means waiting until early afternoon to begin mowing. Sunny dry days are the best times to cut.
Mow your lawn more often to minimize the length of the cut grass blades. This is especially true in the spring during the major growing part of the year. The longer the blade of grass, the more water it contains, and water makes everything stick and clump together.
Always use sharp blades when mowing your lawn. At the very least, get them sharpened or install new ones every season. If you hit large branches, stones or rocks, you have just dulled your blades and should sharpen them. Sharp blades slice rather than tear through grass, and tearing through grass releases more moisture and hence, more sticking.
Clean your deck completely after each time you mow. Use a hose to blast out grass from beneath your deck, and use a flatblade screwdriver to pry out any large chunks that resist the pressure of the hose.
Use a mower deck spray before you mow to minimize sticking and clumping grass. You can find products like this at your neighborhood hardware store that will be a great benefit to minimize grass buildup beneath your mower deck.