Casual under-extruding on Ender 3 (don't know what to do anymore!)
You may have a problem with the nozzle heat setting being too low for the flow rate, which is directly related to the travel speed.

As a background, when you're printing fast, the stepper motor driving the extruder has to move filament more quickly. That's obvious, but what is overlooked often is that the heater may not be able to move heat quickly enough to keep up with the filament.

Consider increasing your hot end temperature by five degree steps until you observe that the under-extrusion is resolved. 

Some colors and brands of filament require adjustment in hot end temperatures. I'm currently printing with silk-like PLA. The filament I usually use prints at 210°C while it was necessary to turn up the temperature to 225°C to prevent nozzle clog.

Not yet viewed in my inbox is a YouTube subscription notice regarding "calibrating your hot end:"

YouTube hot end calibration

I suspect it may be of value to you in your current situation.