I use aptamil and sometimes it is difficult to mix but only on the odd occasion, I swirl it to reduce the bubbles in it too xx
I don’t find the regular aptamil difficult to mix. I add the milk to the hot water though. If I do milk powder first it sticks to the bottom of the bottle sometimes.
I’m not using aptamil but do find Kendamil difficult. I add it slowly bit by bit to a hot shot and swirl it in stages if that makes sense. I don’t shake it at all because bubbles! Xx