It is hard to determine the exact source of the problem as there are few possibilities (I am assuming that you have a single nozzle and only one filament in use):

1. One of the hardest issues for me to get on my printer was fact that my auto-level sensor was mounted about 0.5mm to high - so please check that as this will give you a bit higher Z than expected.

1. Printing speed matters  - for some prints I was slowing my printer to 20% of nominal speed to get adhesion

1. a hair-spray layer or a masking tape on the bed could help with getting the grip

1. also you could play with nozzle temperature de/increasing by 5 degrees and see how it is going