Over time, you may find that the flavour of your vape juice might become a bit dull. This could be because the coil is a bit older, so changing it to a new one should refresh the flavour.
Also, if you tend to stick to the same flavour, it could be that your taste buds are a bit 'deaf' to the flavour and this is sometimes referred to as Vapour's Tongue. Switching to a very different flavour (menthols work really well for this!) for a short while should give your taste buds the wakeup they need.
The last thing to consider is if you've changed from a PG to a VG liquid. VG doesn't carry flavour quite as well as PG, so VG liquids might not taste as strong as their PG equivalent.