Cube single kit, printing problem

Hello, I am trying to print with PLA, at the beginning every thing looks good, but half way through, seems like the the filament does not melt completely and creates thin and loose filaments, seems like a heating problem, this the second time that happens.
Any Idea?, thanks.

Clean the extruder. Blockage on extruder also cause those problem. The instruction for cleaning extruder click on link