Check the fluid level and condition. The lifeblood of the automatic transmission is the fluid. If the fluid is low or it has a bunch of air bubbles in it, the transmission won't function properly. Some people like to sniff the fluid to check its condition. I was advised some time ago that this is not a good practice and can be harmful to your health. You don't need to sniff the fluid to check its condition; usually checking the color is enough, as well as looking for air bubbles like I mentioned. If you pull a dip stick out and there is a rank smell coming from the transmission, you really don't need to sniff the dip stick to confirm it. Go ahead, get it out of your system. I feel like I handed you a loaded gun with that last sentence.
The darker the color, the more contaminants the fluid contains. Healthy transmission fluid should be a nice pink or red color. The darker it gets, the more contaminated it could be. I realize that there are some performance fluids out there that are darker than normal transmission fluid. These fluids still have a nice color even though it's darker than what would be considered normal. What you're looking for is more of a brownish or grey color to the fluid. This could indicate wear within the transmission itself.
I could go on about this for some time, but I think you get the picture. If the fluid is low, top it off and then recheck for the problem you're having. If it goes away, find out why the fluid was low and repair it.
One note on this: On most transmissions, the two dots on the stick are the operating range. As long as you're between these two dots, you're good. If you go to fill the transmission, avoid overfilling. It's much easier to add transmission fluid than it is to remove just a little. You might have to wait for a while between fillings in order to get a proper reading on the stick. It's been my experience that the space between the upper and lower levels is only half a quart; keep this in mind when adding fluid. Avoid overfilling, as that can be just as damaging as not enough fluid.
Also, most transmissions are checked with the engine running and in park on a level surface. Consult your owner’s manual for specifics on how to check and top off your transmission fluid, as well as what type of fluid to use. You might find that on some transmissions you don't have a dipstick to check the level with. These transmissions often require that the fluid be checked from service ports on the transmission. You'll need to know where these are and the procedure for checking the fluid on these transmissions. Also know that the vehicle needs to be level when checking transmission fluid; if not, you'll get a false reading. This goes for both transmissions with a dipstick and those with service ports.