What to do if my landlord will not cooperate with me in allowing me to retrieve my belongings within the 21 days mandated?

Actually under the Arizona Residential Landlord and Tenant Act, in the case of abandonment, the landlord is only required to hold the tenant's personal property for 10 days.  I assume since you stated 21 days, that you were evicted.  If so, you are only entitled to retrieve certain possessions after the eviction, such as clothing and tools of your trade.  If the landlord is not allowing you to retrieve such items, you could sue him/her for the return of those items, but that may take too long.  You may seek the assistance of the police with retrieving your items.