Possible cause:

1) This can happen if you protect content under a different product but not under the product to which the user has access.

User can only access protected content that is dripped under the product(s) to which the user has access.

Say you protected http://yoursite.com/members under product A and not under product B.

Then a user that has access to product B cannot access /members because it's protected under a different product (product A) and not under the product or products to which the user has access.

2) NOTE: this does not apply to users with DAP 4.3 so skip this step if you are on dap 4.3 or later version of dap.

Check if the user is marked as free user in manage users page. If yes, check the content drip / protection section and see if you made the protected content available only to paid users (isfree = N). If yes, free users cannot access it

3) Make sure NOT to use any CACHE based plugins like WP SUPER CACHE or WP TOTAL CACHE as it can impact content protection / login etc.