Trying to obtain an increase in rent on a regulated tenancy

Other

I have sent the necessary form to the tenant asking for an increase on his regulated tenancy rent. There was no response to this form from the tenant so i checked with the post office as i had recorded it and they informed me that the letter had been collected. I have tried other ways of contacting the tenant i.e. e mail and telephone but still i have had no response. I also have contacted the rent tribunal and he has not contacted them either to dispute the rent increase. What would be my next step please? Regards Debbie