As long as others are still trying to catch, which keeps the instance alive, you should be able to restart the app, rejoin and catch it.
If the boss is dead, and the instance is still alive, you can rejoin, and you will get your rewards, and then be able to catch it.
Most times it requires that you completely restart the app.
Once, I even had to restart my device and then could join - but I had a friend wait to catch the boss until I was in again.