Even if you catch your opponent in a triangle from the guard, a tap is not guaranteed. There are a number of counters and escapes from a fully locked triangle choke.
Here are two videos on techniques to deal with two common ways your opponent will counter your triangle choke.
The Arm Wrap
The opponent has wrapped his arm around your upper thigh and may even secure a strong grip on your belt. In this short video, Professor Gui Araujo shows us how to counter it.
Bigger opponents especially will pressure forward and stack you. It is an uncomfortable position and you can not generate the leverage to get the tap. However, there’s an easy way to counter it: change the angle, sweep, and switch to a triangle arm bar.