Do franchisees have legal obligations to pay royalties to franchisors on loss-making product introduced by the franchisor?
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Having said that, the situation you describe doesn't make much sense--especially if it's a long-term problem rather than a short-term factor. Most franchise systems have advisory councils of franchisees to interact with the franchisor when there are fairness issues that need to be addressed, and most good franchisors are willing to listen and adapt to systemic problems affecting franchisee profitability.
I would advise you to first make sure this is a systemic problem and then run it up the flagpole of exposure so the franchisees are collectively addressing this with the franchisor in order to have more bargaining power in terms of getting this addressed. I have seen this approach work effectively and constructively many times in the past.