Non rent payment/new agents

Rent Arrears

Hi, I am still a relatively new landlord and the last year has really tested me.
The agents that I had managing my property sold up to a larger well known agents without tell anybody.
Now the tenants in one of my properties hasn%E2%80%99t paid their rent. They didn%E2%80%99t pay Nov, Dec or jan.
When I raised it with the agents to find out what is happening I received one payment. However now they have missed February payment date. What can I do about this?
I have rent guarantee but the agents seem reluctant to use it.
I%E2%80%99m sorry but I%E2%80%99m pretty new and naive to what I can do about this.
As the new company isn%E2%80%99t the original company I also signed a contract with, would I be able to employ another agents to manage my properties?

Thank you