So my old technique for changing areas/finding somebody in a dream was to fall backwards through the floor, sink into blackness and then fall to my destination.
However, the last two times I’ve tried this, the moment I’m about to sink into the ground, I get this immense pressure and pain in my chest, and I wake up.
Any idea what this could be, how to avoid it or other techniques for finding characters/places?
Sometimes, for whatever reason which only you could possibly know, stuff which works usually in dreams doesn’t work. I’d recommend not stressing it so much and just trying it again and again, while being confident that you are in control.
As for other techniques, I usually summon characters by either just calling them or reaching out my hand behind me, expecting them to grab my hand. To change places, I expect a door behind me, turn around and open it with the destination (or person) in mind. Another thing that has worked is walking through a mirror or to summon a portal, but these are just variations of the door method. What I’ve used only once (but which was successful) is telling the dream to take you to person or destination XY, when I did this a strong wind or force pushed me from behind and took me to the place in no time. Belief is everything! 
I interpret this as being out of harmony with your subconscious. When you’re lucid and everything flows smoothly, it’s not only because you are lucid and excellent at being lucid, but also because the dream realm cooperates. Not as a separate thing: your lucid self is also able to intuitively understand what will work, and what won’t work, and why.
When the conscious and subconscious are not feeding into each other, like a nonmoving yinyang, then control fails. I determined to fly through a door, knowing that assassin dream characters would be forced to open the door manually, and even though I was fully lucid and had full confidence and no doubt… I crashed right into the door, as if it were solid.
I believe that the door represented my waking life familiarity. If I maintained both a psychological and situational obstacle from breaking out, perhaps a fear of change or the unfamiliar… then, I will dream of a door that I can’t fly through.
If I feel weighed down by pending waking life demands and such, I might lose my ability to fly in lucid dreams, because flight means freedom to me.
So, the place that you couldn’t escape from might have represented something. Or, floors might represent something to you.
Thanks, guys. You’re right about belief, it probably happened the second time because it happened the first time making it the most recent memory of using that technique.
Elly, floors or the area I was in don’t have any particular meaning to me. Then again, if I think about it, I suppose that both of these times the ground was concrete rather than dirt/carpet. Maybe that has something to do with it.