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Unfortunately, sometimes fleas become immune to flea medications so without knowing the name of the medication I cannot look into this specific issue further and determine if this one of the medications to which fleas commonly become immune.
Another likely scenario is that there are new fleas coming in from the environment that are biting your dog. Some pills work by killing the adult and homework by preventing them from reproducing. Either way, the flea generally needs to bite the pet to be affected.
The topical products are generally the ones that kill fleas without them needing to bite. If your dog is allergic, I recommend a topical product in addition to the pill to prevent the biting as this is what causes the allergic reaction and discomfort.
Weather patterns and season can affect flea populations. Either way, if she is experiencing an infestation, then addition of a topical product may help her. Its possible that either the fleas have developed resistance (their generation time is much faster than ours so this can develop rather quickly) or that for some reason its duration of action is not as long as the ideal.