I ran into a problem similar to this, where the print would start fine and then after 10 to 20 minutes it would be air printing. The filament was grooved by the hobbed bolt and would no longer feed. The amount of retraction made no difference, and the hot end temp made no difference. After multiple tear downs and re-assemblies I found the problem was the cooling fan (which I also have turned to blow out rather than towards the fin area). The fan was working, but it had slowed considerably and was not providing enough cooling. The heat was traveling up the filament and making the filament to soft at the hobbed bolt. Replaced the fan and everything prints perfectly again. Hard to diagnose, but easy to fix.

The cheap fans that come with the MakerFarm just don't seem to last well. If anyone knows of a quality maglev fan to use as a replacement I would appreciate a link, but until I find a better one I know if the filament is being gouged by the hobbed bolt, it's time to change the fan. It would be great if a future firmware revision could add support for a 3 wire fan and could show fan rpm on the LCD. That would be a big help in determining when the fan is slowing down because it's not really obvious just from looking at it while it's running.