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Why do password protected pdf files not work when using sumatra.

If I use another pdf reader the password works?

There could be many reasons, but without a sample cant tell.

First which of the three types of passwords do not work?

However since you are saying other apps do then I am guessing it is connected to OPENing the file (usually called user password)
The third type (here I am excluding any password related to signing) is called the Owner pasword that controls the users rights, thus can be confusing. any way many reader apps just ignore that one so its not worth adding it.

There can in general be two types of user password to OPEN, one is the conventional sort that should work on any pdf reader (unless there is some special unexpected character). Again it is more trouble than it is worth as many online and offline tools will crack the simpler 40bit forms, and even the tougher variants will eventually get broken.

The other type is to open a dedicated DRM version of encrypted file that becomes a decrypted pdf once opened. They either need a modified plugin within Acrobat or some close variant that supports plugins like Foxit or else they have their own modified un-wrapper. They usually charge to protect the file from being copied, but work on the basis they show the contents on screen, and as soon as you do that then they become useless. At the easy end you just Print/OCR the visual output, or download the PS objects from memory.

Do not get me wrong, they do work to make money for the seller, I say they are a waste of the users time (and money) because, first you waste time setting them up, then the user wastes time whilst they often slowly open the contents, then at that point the user can do what every they wish with the contents, so don’t believe the hype that they will protect your copy, you pass owner rights over as soon as you give a user a full permission password to open and thus decrypt content.

Know it works…Sorry for wasting everyones time.

Can/should I delete this post somehow?

Glad it is working as it confirms there is possibility of “finger” or Memory troubles, often CapsLock or other shifted characters may be affected by a sticking key.
It is helpful to leave it for others to raise related questions, as there are other related issues, such like you can store regular passwords to avoid keying in each time.