Frequent requests for Log on recently

For the last few weeks, I have tried to post a reply in a thread and several times i have been told to log on. On several occasions, when I did, I was told my password was incorrect and asked if I had forgotten it.
I went to the forgot your password? item and was given a temporary password to re-set and have put in my old password and been accepted.
Why does this occur? I have been a member for awhile with a modest amount of posts, but something is happening.
I also need to say that on my iPhone, I have to log on with every reply.
Help! i enjoy TD, especially the outdoor board.