Just received a letter to say I'm in £120 worth of rent arrears. I'm meant to be in receipt of full…

Just received a letter to say I'm in £120 worth of rent arrears. I'm meant to be in receipt of full housing benefit and they know I am a lone parent, everything has been declared and proved. Does anyone know why they might have stopped my HB or possibly reduced it? I don't want to lose my home they're threatening to evict me now.