In-Page Links

Hi everyone.
after checking bhw for a while but no result i decided to open a question thread so maybe someone can share his wisdom and help with In-Page Links. lets say that i have main keyword: service x product, and other keywords are service product model 1, service product model2, service product model 3 , service product model4 and service product model 5. on all 5 models i am serp 1 or two, but on main keyword i am ranked 6th. how can it be? all secondary keywords contain main keyword but only thing that changes is the model. service + product is the same.....
please someone share his wisdom with me because for me its a quiz that i cant solve.
rep incoming ofc

Its normal, service product model 1, 2 etc.. are long tail and easy to rank in all kind of niche.

try to find more relevant keywords to increase traffic and diverse your anchor text in link building. you will also rank slowly for main keyword.

in site contents for long tail keywords and also anchor main keyword in these contents.

Hope it help.

Yes your longtail seems to be ranking, if you want your main keyword to rank more try adding more related content.