I´ve seen this in my Prusa due two parameters that may vary your results depending on climate if your printer has not a temperature chamber or having a mechanical issue too.
Lack of extrusion is due a cold filament which it can't reach the melting temperature due a fast extrusion feed; I mean in normal conditions we can print @70 mm/s with 195 °C but on wet or colder days is not possible so I need to slow down the speed (feed rate) with 10 % less than normal to get @60 mm/s or less until get a good flow with out modifying the G-code. If I try to print faster on normal conditions I will get the same lack of material due 195 °C is a low temperature (this is an example).
If I set the temperature 200 °C or 210 °C I will get a better flow and also print faster than @80 mm/s (not affected too much on climate on 100 % feed rate).
For first layer I´m using an speed of 40 mm/s to allow a good adhesion and Z height 90 % of layer height (0.22 typically or 0.18).
Mechanical side: The extruder is not feeding all the filament due a missing pression on the traction gear (filament slip).