As I train BJJ, over time common themes appear as I roll. Recently one thought that has popped into my head while rolling and securing a choke is that human's can be extremely stubborn. Particularly training partners when it comes to chokes. With joint locks people tend to tap quickly, and rightly so, because the result can be catastrophic if ignored. However, with chokes I've noticed that sometimes it doesn't feel quite in the correct position. It might be uncomfortable but the guy is not tapping.
When this happened in the past I would let go because I felt the technique was not perfect. You should realize by now, there is no perfect technique, just perfect timing while executing the move with a good level of technical ability. When the choke is on, hold it and allow it to sink into the opponent's neck. This doesn't mean that if it's not working, do it stronger and with more force! It means relaxing every part of your body except the choking grip and let it slowly do its job. Trust the technique.
Most of the time the opponent will eventually tap as their neck is no longer able to cope with the force of your arms or the kimono shrinking around their neck like a boa constrictor. A good rule of thumb is when you think you should give up on the choke, hold it for a count of ten. If it doesn't work even after that, move on to something else. If you can't hold it for 10 more seconds, work on your squeeze by doing pull ups.
Be Persistent with the choke. Especially when gravity is on your side.