Did you check and make sure they don't have an existing connection that's froze. I'd delete some of the pending instant clones in the horizon administrative interface so new ones get created, and make sure the user doesn't already have a session, if so you may need to delete that one so they are kicked off and able to use a new one..
Yes i have checked, there are no pending connection. I even restarted all the available machines so that a fresh machine is available to connect. but still the user is not able to connect.
Could you confirm which protocol are you using?
Are you having issue with both internal and external connection?
Are you using UAG or and Load Balancer?