The phenomena has a technical term “Dirty plate syndrome”. 
But seriously, don’t overthink this. Take the plate and wash it with dishwashing detergent. In the US, Dawn is best and in the EU it goes under the brand Fairy Liquid. Smear a generous amount on the plate, run your kitchen sink at the hottest and most forceful amount and spray off the soap until it’s clear and repeat. Then pat dry with a paper towel and try it again.
Another alternative is to increase the bed temp by 5-10 degrees. But if the plate has any contaminants, it will only produce limited results.
A very effective test to determine how clean you plate actually is, is a first layer test. This can be done by taking a cube primitive, scaling it to 250x250x0.20mm and center it. That comes to one layer of filament on a 0.40 nozzle. If there is any contaminant on your build plate, it will become extremely obvious like in your photo.