There is no simple explanation why Martial struggles. It's a combination of several factors: personal weaknesses (I wrote about them last season and was criticised by his fans) and Jose's inabilty to work with young talents (it's not like Rashford has progressed under him, not to mention Shaw).
Rashford is getting decent chances considering his age, always used as a starter or a sub. No player at 19 years old will be playing as a regular starter unless he's a freak of nature and Marcus is getting what a kid at the age of 19 should get. Regarding progression that depends on what we're you expecting to see from him to consider him "developed" . imo he's progressing steadily as any 19 years kid would and getting experience over time.
Also when Jose is giving multiple chances for Rashford like this it's strange to consider him unable to deal with youth or don't want to give them chances. Why is Rashford always in his plans ,either a starter or a sub, while Martial is struggling to see the line-up ? Can we ask ourselves this question instead of just blaming the manager anyway??