If you have a Heng long black 27mhz transmitter will not light up ,the issue will be that the batteries are not making a good connection.
Some batteries are slightly different sizes and so if you remove the spacer tabs in the centre channel of battery compartment at the rear of the handset this will fix your problem as these tabs are made too fat, this holds the batteries too far apart so the handset does not light up ,
With some sharp side cutters you can just remove these tabs and the handset will work fine.