I bought a second-hand Nivona 830 coffee machine. When I bought it, I checked that it was working properly by making a single latte. At home, I wanted to make two in quick succession, but it’s impossible. The first one comes out fine, but with the second one, the milk sprays everywhere and doesn’t froth – only a little bit comes out. After trying again half an hour later, the same thing happened: the first coffee came out fine with frothed milk, but the second one, unfortunately, didn’t.I’m wondering what’s wrong.