You very likely have partial clog in your hotend (the side which is clogged is the angle the extruding filament bends to) or have some plastic somehow stuck on the nozzle surface which the extruding plastic almost sticks to, like two droplets of water merging into one.
If its the latter, clean the hotend. It's easier to remove the plastic on it while its hot and soft, but don't burn yourself.
If it's a clog, that's more complicated.
First, you can make sure if it is indeed a clog or not by extruding into the air and measuring the wire with a caliper.
If it's thinner than it should be and your slicer settings are fine, then its a clog.
Ways to get rid of clog:
1) Replace the nozzle with a new one.
2) Break the clog with a sharp wire.
3) Take the hotend off and clean the nozzle with acetone ("acetone bath").
4) Blow torch on a removed nozzle. Dangerous thing to do for some but it works. If you're not confident or experienced to do this and the previous two solutions don't help, then go with option 1.