This may seem like a stupid statement, but make sure to pull the stick, put it back in, and then read. If you just pull it out of the tube (even if the engine is warm and you have run it through all the gears briefly) it may read dry. The reason is that there is a very good seal on the top of the dipstick tube. When driving, the fluid level is very low (low in the pan) and the dipstick tube is almost dry. Fluid doesn't get into the tube until you pull the stick. This can change depending on how hot the fluid is and how good the seal on the dipstick is, etc. It can appear as if the level is very inconsistent.