civic sir2

when i warm up the car the temp is where it should be in the middle then after 15- 20 mins the fan doesnt kick on the temp goes up then radiator fan kicks on after a while then shuts off car temp goes back up its like the fan isnt kickin and stayin on when its suppose to. i got a new radiator, theremostate, coolant is flowin right its just that the fan doesnt kickin on when its suppose to and when it kicks on how long should it stay on??? cause mine doesnt stay on that long if anybody knows wat it could be let me know appreciate it!