When I sit for Vipassana and observe the sensations, they intensify. The sensations get stronger and stronger the more I observe them. After a limit these sensations break my will power to sit and force me to stop. What is happening here, can anyone explain?
Normally our attention is diffuse and weak. When you focus on anything it becomes clearer. That's a part of it for sure. The same will happen to sounds, etc. if you focus on them. Of course, pain will increase itself if you don't take steps to alleviate it, so that's often also a part.
Important is that the pain is not a problem, and if you can see it just as pain, it doesn't matter how intense it becomes. The Buddha said we should bear with pain even if it feels like it will kill us. When you are averse or afraid of the pain, you should take that aversion or fear as an object of mindfulness. This will help you see clearly (vipassana) that the pain is just pain:
dukkhaṃ vā vedanaṃ vedayamāno ‘dukkhaṃ vedanaṃ vedayāmī’ti pajānāti.
When feeling a painful feeling, one knows fully, "I am feeling a painful feeling."
The idea is that that should be all you know about it, not that it's bad or a problem or getting worse, etc. All of those things are reactions and should be taken as objects of mindfulness as well.