Costs may be disallowed if you pay the sponsored party and there is no clear business benefit to the company.
You should not sponsor your personal hobby, as there would be a duality of purpose. Similarly, any payments to relatives under the guise of sponsorship could be deemed to be providing disguised earnings to them, therefore the related sponsorship costs would not benefit from a tax deduction.
Sponsoring an event unrelated to your customer base or business activities could also be a reason.
Failing to explore other sponsorship options or not reviewing the impact on your business trade might lead to disallowance.
Example: If you agree to pay any amount to a sponsored event request without discussing the value for your business, HMRC might disallow the expense.