Why does the automatic transmission fail to engage in the morning?

After the vehicle sits overnight, in the morning when you start the vehicle you can put it in reverse and back out of the driveway, it works and drives fine, when you shift to drive and step on the gas the rpm's go up and the vehicle barely moves for about 30 feet. After that the transmission appears to catch and the car drives fine for the rest of the day. This includes turning off the engine and restarting the car several times during the day. This problem occurs each and every morning after sitting overnight. I have had the car to a transmission shop, they checked the fluid level which was full, checked the color which was red and still looked good with no smell, and checked the codes none of which were found. They stated they have never heard of anything like this before. Do you have any ideas as to the problem, or a possible fix? Thanks!