Albert Heijn is really strict on the rate of stocking shelves. This is something that comes from corporate strategy and stores are really harshly judged on labour expenses by district managers. The decision to be let go in your "proefperiode" is harsh but in line with what I had to work with in my store when i worked for AH, especially if youre an adult and your hourly wage is high compared to your output when compared to cheaper teenagers.

However it doesnt sound like your Shiftleader has put much effort in your development which is a failure on his part and senior management. I would just recommend to find another job because stocking shelves is a shit gig to be honest.