The subconscious mind is a vast and complicated conglomeration of memories and feelings both past and present. If your subconscious fears loosing love, approval, safety or security by getting rid of the problem, the subconscious will not let the issue go.
For example: somebody really hurt you and you can't seem to get rid of the unhappy feelings. Your subconscious is holding onto these feelings to remind you that this person is dangerous. If you get rid of the anger or resentment, you could loose safety.
I have heard from a few that EFT does a poor job on allergies. Allergies deal with a lose conflict. If your subconscious does not want to lose whatever it was, then it will not let the allergy go.
Also you must be very specific with EFT. The more specific the better. A young man with allergies to dogs had no recollection of a lose conflict with dogs, but his mother recalled the city dog catcher catching his dog on the street. This little boy was very distraught, crying for days and yelling at the dog catcher whenever he saw him.
We did EFT on each aspect of his lose conflict and his allergy disappeared almost instantly.
Often tapping on symptoms does not work we need to be as specific to the original allergy causing event.
I worked with a teen boy with a milk allergy, he was born with it vomiting his mother's breast milk as soon as he was born. In this case the lose conflict occurred to his mother while she was in the 6th month of pregnancy.
We did EFT on the lose and feelings of rejection his mother experienced. Afterward he was immediately able to drink milk.