When you log in, do you check off the remember me button?
Also, there is a session timeout implemented within the forum. If you are logged in until that certain time, the forum will log you out automatically.
This is for security reasons.
Yes, I do check the (remember me) box when I log in each day. Once I have previewed posts I want to see, I leave the site via the X in the upper left corner of the screen. I presume that this indeed (logs me out). If I return later on in the day or evening, I'm still able to get back into the forum without going thru a new sign on and pw. drill.
But the next day, It will not remember me and I have to go thru whole process again. Not sure if this relates to your (time out) comment and security reasons comment.