Check the espresso recipe and grinder settings regularly. When i went in the dose was 16g's (should be 18.5) and extraction time was 10 seconds. This will create a weak tasting, under extracted espresso, resulting in a sour / acidic taste through milk.

Your espresso should be extracting in 25-30 seconds to create a balanced, nice tasting coffee =)

I have suggested a Grind by weight grinder to Kay, these grinders do not need as much input from a barista to dial in as they have built in scales which consistently give you the correct dose of coffee each time. This would improve the quality and consistency of coffee served