also, check to make sure you actually have air transport units, you need min 25 military transport plains for one ATU. you probably checked that, just throwing it out their. the more you have the faster the drop/lift, i try to make a point of having each country to have 100 fully stocked ATUs. but i do ALLOT of airlifting.
also, if your unit is in enemy territory, unless you have already dropped, and completed the RDU, it wont let you airlift out. the unit has to have a freindly, or owned airport in the country that is being dropped/lifted FROM.
shrug, just some things to check