I was named manager of my department and told I was to be in charge of my 'realm'.

I was named manager of my department and told I was to be in charge of my 'realm'. All department managers get a say on the hiring of assistants for their departments and they even get to interview their future teammates. It was deemed necessary for me to also have an assistant. I was assigned a colleague to help me who was also being trained for a different role by my supervisor and eventually take over my supervisor's duties. It has been over 6 months where I have to keep reminding colleagues to help me. I have spoken to our direct supervisor about the situation multiple times and was reassured she would discuss issues with the boss and colleague. Each time manager speaks to a colleague they will start helping for a couple of days and then again to MIA. One time, the supervisor, the boss, and I had a meeting to discuss how to grow my team and results and I brought up this situation to which the boss said this was the first time he had heard about this problem through the supervisor had promised to speak with him about the situation since the colleague was also having issues completing projects for supervisor. Any advice? Recently, we have been appointed a new boss.