Originally Posted by
m. zillch 
The blast of air works on two principles:
A) Cleaning away any small layer of dust which is blurring the laser's output or reflected view of that output.
B) Forcefully smacking it up side the head, similar to how one would kick a vending machine if it fails to dispense your soda

. Your are hoping to dislodge something which is sticking. The thing is it is a delicate assembly which has to precisely focus on a surface which is so small that it is invisible to our unaided eyes.
If, let's say, your camera was having troubles , would you ever think to smack it ? Well oddly you may actually fix your camera in some instances by doing so, but do it by the wrong amount, a very hard thing to measure or describe over the web, and you may very well break some other aspect of your camera!

I guess to clean your lens more gently you could also dip a Q-tip in 99% alcohol , shake off the excess, and gently swab across the laser surface a few times, allowing it to dry before playing a disc, but I can't say I've ever done this.{ And I take no responsibility if anything goes wrong. } Others on the web talk about cleaning lasers in this way, for example Playstations, but "dirt/dust" may not even be your issue. It very well may be laser focus or angle, both which can be affected by heat build up which is supported by your observation that BD play fine for the first part of the movie and then get stuck.
You've exhausted my knowledge on the topic. Good luck.