This could be down to the user not priming the coil correctly, the liquid used or the way that they vape.
If you take very long draws or a few in quick succession it can exhaust all of the liquid in the coil which is possible even with a full tank of liquid. The liquid in the tank re saturates the coil after each time you have a go on it but this does not happen immediately it can take a few seconds to fully saturate the coil again.
Ultimately if it tastes burnt it is because it is burnt. This can be the cotton was heated whilst dry before it has saturated or there is a build up of residue from the e-liquid from regular use.