Hi,
Have you had a look at the NPN datasheet to see if anything related to hFE is affecting it? The collector current is meant to be 1mA or 13mA? - Is that a 2k2 or 22k resistor? What's going into the base, 2mA? Is the base high voltage around 3.5V, and is that enough to fully turn it on?/Have you measured the "high" voltage coming out of the 555 output pin? What's the load resistance/current requirements? Maybe the 555 output voltage is lower than needed/expected at 40kHz.
The load is powered when the NPN is off, and the load is bypassed/off by the NPN when it is turned on, isn't it?
It may also be that the timer output voltage just isn't enough to turn the transistor on and you would need to cascade two transistors.
You may need to provide more information about the BJT and the transmitter, or some measurements/oscilloscope observations for anyone to take the time to answer, I'm afraid.
I'm not an engineer, but I've at least tried to help with constructive comments and what I know a little about. Hope you've solved the problem, anyway.