There can be a number of reasons why this is occuring! Take a look at the following possibilities/solutions.
You may be experiencing under-extrusion. In your slicer, make sure you have selected 1.75mm and not 2.85mm
You may have microclogs that are causing the under-extrusion. For this, you will need to disassemble the hotend, and clean the Bowden gap, the ptfe liner, and the rest of the hotend to eliminate any possible clogging.
The extruder drive gears may be the cause as well. They may be too loose. Please perform a 100mm test. Measure out 100mm from the extruder, mark that length with a Sharpie, heat nozzle to operating temperature, ask printer to feed 100mm of filament, wait for it to finish its cycle, measure distance remaining between the extruder and the Sharpie mark. This distance will tell you how much you are under extruding. Anything between 0 – 5mm is acceptable and common (factory tolerance). If the distance exceeds this range, look for binding in your Bowden tube and gears, increase tension on the extruder(s), and reperform this test multiple times until you reach the acceptable range. If you have recently changed the extruder drive gears, you may need to adjust your M92 settings in your firmware.