Trying to serve S21 but agent is creating problems

Possession - Section 8 and Section 21

I had given my property to an agent to manage. It was first given in August 2017 for 6 months and ran into a statutory periodic tenancy by not doing anything until 1st May 2018.  In May 2018 the agent increased the rent by £50 and until today there has been no rent increase what so ever because the tenant has flatly refused a rent increase saying he cannot afford it, he has 3 children and that he is only a cab driver.  

The agent has always been on the tenants side sympathising with him and saying he is a good tenant.  According to my calculations I have lost about £20000.00 in the 6 years and the agent does not care.  The agent has not given the prescribed information.  He has deposited it in My deposits and have sent me their online portal which clearly says the deposit was lodged.  I did not query that.  My query was why the prescribed information was not given.  They were shouting saying they are professional people and their legal team says they have done everything right etc.  I asked them to return the FULL deposit to the tenant.  They insisted on leaving 5 weeks rent and returning the rest.  They are saying they serve S21 all the time, etc etc.  

I think they want me to fail S21.  The latest is they are saying the tenant has to agree to get the deposit back and has not sent the bank details to 'My deposits'.  Now according to them the tenant who could not afford a rent increase does not even want the deposit back!  What a joke!!.  Is it true that the tenant can refuse to take the deposit back? What do I do under the circumstances?.  The agent acts as if its their property.  Today they were saying I dont seem to trust them and to come and collect the keys.  I cannot deal with a crafty tough tenant.  My husband has parkinsons and a lot of other issues.  Can I give it to another agency at this point in time?.  Will another agency accept a case like this?.  I feel totally lost.  Can someone please advise me what I should do.  Now my problem has doubled by giving it to an agent.  Any good ideas will be welcome.  When I phone the land lord line very often the line is not clear due to a combination of my hearing problem and the fact that calls are recorded which ofcourse they must do and I totally 

they have to do that and I can understand that very well.  I tried to phone Land lord action solicitors but they said some one will phone me on Monday.  They seem to be too busy.