Is there a lot of lag?
If not, try double-clicking the tech you want to switch to. Remember that it needs a cab to do so. If there is too much lag, start your game over and attempt to change the graphics to the lowest quality needed. and when gameplay is smooth, try again.
Hey Pretzel, I've had similar issues and as WhitePaw said, if there are too many things happening it will lag the game and will give you a hard time switching techs.
In my mind you can try one of two things. One, reduce the number of silos that have resources floating in them if you have that, or reduce the number of repair/shield bubbles. For some reason the game insists rendering these items with nice glossy/shiny effects which will lag your game.
Two, (and I think it's similar to what Reaper said) you can try and buy a standalone cab, drive it to the tech you want to switch into, and attach the cab to that tech. This to my understanding will make you control the tech at which point you can remove the redundant cab and store it or sell it. I've had better luck switching into just the standalone cab vs trying to "click into" a complex tech.