As Ca Putt said, you need to remain in control of these provinces. For the unconditional Marian reforms, you need to control no fewer than 90 settlements. You should conquer 3 more provinces and hold the settlements.

Also, they will not trigger if you have not fulfilled the requirements for the Camillian reforms (owning 2 out of the three Sicilian provinces, and 2 out of 4 of the North Italian ones, if memory serves).

Once you do fulfill the requirements, reforms should kick in the next turn. Don't think there will be a delay of the reforms triggering, since it is perfectly possible to bypass Polybian reforms completely (to go from Camillian to Marian, you would need to hold 90 provinces + fulfill the requirements for the Camillian reforms).