No agreement when faire is used as a past participle

Question

I got this question wrong in a test on another site. Unfortunately no explanation was given. Can someone tell me why there is no agreement when faire is used in this case as a past participle ?
 
Les modifications que j'ai faites apporter au projet sont approuvées. (Marked wrong)
Les modifications que j'ai fait apporter au projet sont approuvées. (The right answer)

Accepted Answer

Ian, 
the attached links to discussions of the causative structure with ‘ faire infinitive ‘ should help.
 https://www.lawlessfrench.com/grammar/faire-causative/
 https://progress.lawlessfrench.com/revision/grammar/faire-plus-infinitive-means-to-have-something-done

Answer

Bonjour Ian,
Maarten's answer and suggested links are great!
Generally, with the causative "faire", it will always be invariable: 
Agreement with "Faire + L'Infinitif = to have something done (causative)"
Also, here, the direct object "les modifications" is not the direct object of "faire", but of the infinitive "apporter" (apporter quoi ? -> les modifications) This is why "fait" doesn't agree with "modifications".
I hope this is helpful.
Bonne journée !