After switching a flavour in your tank, you may experience tasting the previous flavour that was in your tank. There are a couple reasons this could be happening.
First, some of the old vape juice may still be in the coil. Coils have cotton in them which absorb juice. Since your coils is already saturated with your old juice, it may take a couple of puffs for the coil to start absorbing the new flavour you put in. Just vape for a little and you should notice the new flavour coming through and getting more flavourful as you puff.
Second, if the previous solution doesn’t solve the issue your coil may be stained with your previous flavour. This is notorious for mint, tobacco, cinnamon and heavy dessert flavours. The cotton in the coil tends to retain the flavours. Unfortunately, every puff you take will taste like a hybrid between your current and old flavour. To fix this, you have to change your coil and start with a fresh one.