My Car Won't Go in Gear

Vehicle Problem: I Have a 1998 Chrysler Seebring. Occasionally it does not want to go right gear.I just had the shift solenoids replaced for the same reason. What could it be? Solution: While test driving the vehicle I quickly notice the symptom it was brought in for. As I put the shifter in gear the vehicle did nothing until I reved the engine slightly and then it began to move. I drove the car around the parking lot and stopped with the car still in gear. Waited 30 seconds and had to rev the engine slightly to get it to move. This confirmed the symptom the customer was having. I left the engine running and popped the hood. I removed the transmission dipstick and proved my suspicion, it was not even registering on the stick. After adding fluid, in the amount of two quarts, I let it settle and double checked the fluid level. Test drove the vehicle for 20 minutes around town and had no more issues. Triple checked the trans fluid level and all was well. Raised the vehicle to see where the lost fluid might have gone since the transmission is a completely sealed unit. Cleaned off the bottom of the engine/transmission area and ran the engine. No signs of leaks found. Drove the vehicle around town for another 20 minutes and recheck underside for leaks, still no sign of leaks. I can only assume that the dealer that had replaced the shift solenoids did not top off the transmission fluid level. Customer is now happy !! Cost to the customer was $94.00 plus tax... This is an Actual Situation that occurred on January 1st, 2008