User id not set sometimes but fine sometimes too

I have forms where I use the registered user id and insert it again any other data that is entered by that user.

USERS_LISTINGS_INPUT_DATA_LogIn.php

I have used eCart to set session id = USER ID.

However when some users login in they enter data (i.e. a venue) and have their user id '80 or 800 etc' against the data with the insert and update date.

Other users login is and enter the same data and have a user id in the users table but that users id doesn't populate always in the following forms

So in a nutsell inserting the USER ID from the Session ID works sometimes but not other on the following pages:

90% of customers are fine, 10% of users have [null] in the DB for the User id next to their listing data, their meeting room data, their rates data and their spa data.

LISTINGS_Insert.php

MEETING_ROOMS_PER_LISTING_Insert.php

RATES_BEDROOMS_PER_LISTING_Insert.php

LEISURE_SPA_FITNESS_Insert.php

Can you see why from the code please?

Thanks,
Neil