This article was helpful in formulating my answer, and is a good context for those out the loop.
The issue here is circular; the Native Africans of Rhodesia aren't going to accept the agreement because the negotiations didn't involve them. The negotiations didn't involve them because Ian Smith didn't want them to be in the loop. Heath couldn't force him to involve the Africans, so Smith didn't. And thus, because Smith didn't have to and didn't want to have the Africans involved, they didn't agree to the negotiations they weren't represented at, and thus rejected the agreement.
For the agreement to work, you need to have Smith either be forced or agree to bring Africans along with him, so they are part of the agreement and can accept it. You also need Heath to be able to project power to enforce the agreement, which in 1971 he couldn't do; Heath couldn't intervene because of the weakness of the military, and because of the opposition to intervention from Third World States and Commonwealth members. So, responsibility of the working of the agreement must lie with Smith inviting and bringing African representatives to the agreement, and had he done so, would have been able to nip the War in the bud.