Really... The problem could be anything. But it looks like you are not getting enough filament to come out. It could be your hobbled bolt teeth are filled up with stripped plastic and are not biting into the filament well enough. It could be you have been switching from PLA to ABS and back and the inside of your extruder needs to be cleaned and flushed. It could be dust has built up and is not totally plugging the nozzle, but is restricting the flow a little bit.