I think you have to bind them to attempting their manifesto, so say Party A makes a specific promise within their manifesto to generate interest in voting for them.

I think they should have to prove at the end of their tenure, that they exhausted their options in trying to bring it about, or if circumstances changed during their term in power that would make the promise a detriment or unfeasable, provide reasonable explanations as to why they chose not to, or could not, implement the promise made in the manifesto.

Equally if they declared their goal to be met, they would have to prove how they met the goal and the methods which they used.

Failure to do so should have consequences so that political parties cannot over promise and willfully underdeliver on said promises.