You say it happens evey time with a new pen (I assume you mean regardless of the brand). From what I can see you are applying quite a lot of pressure when writing. I don't know if it is possible, but perhaps this excessive pressure may cause deformation of the point where the ball gets imbedded deeper within the socket, resulting in less ink passing through. You could get a new pen, try applying a bit less pressure when writing, and see if anything changes.
When my pens start doing that I try the following, in this order:
-Write squiggly lines on scrap until the ink starts to flow properly again. Sometime's ink dries on the ball, this is a way to remove it.
-Hold the pen by the end opposite the tip. Twist your wrist back and forth swifty; the goal is to force the ink down towards the tip using centripetal force.
-On SOME kinds of pens, you can blow into the back of the ink reservior; same goal, force the ink towards the tip, but using pressure this time.
-Last option: use safety! I will apply a lighter or match flame to the metal tip of the pen for a few seconds. Be careful not to melt any plastic. This one is particularly useful if the ball gets deformed and stuck, such that it cannot roll. The heat will expand the outer casing of the ball-point, allowing the ball to roll again. If the ball is TOO deformed, this will only work temporarily.