<p>Every time I start a print, midway during printing, my printer starts to under extrude.</p>
<p>I tried lots of different models every time the problem occurs.</p>
<p>What should I do about this?</p>

If everything is stock on the machine and all other setting are properly configured, this is most likely
heat creep
.  This is caused when the PFTE tubing is not properly set within the hot end itself leaving a small gap.  As filament is fed into the hot end and heated to melting, some filament oozes out in the gap between the PFTE tubing and the nozzle. This melted filament then cools and hardens making it harder to extrude filament causing under extrusion.
You may want to calibrate your E-steps and flow as well,
here is an excellent website
.