First, lock the plastic knob in place for the correct dose, and then press hard. It seems to require a firm push each dose. Just make sure the plastic lock knob is fully locked into place first, so that you dont empty the entire tube by accident.

And for covid, I take it after consuming a meal having a high fat content (30-40 grams fat) such as from foods like cheese, cheese pizza, spoonfuls of mayo/peanut butter etc.