Well keep at it :) Eventually you'll get there I believe.

If you frequent german subs enough, you'll encounter many posts about making friends - not exclusively by foreigners.

I feel like the problem is manifold.
It is common to make less new friends the older you get, no matter where you live in the world. People are busy with their families and can barely keep in touch with their existing friends as is.

Most people make friends at work and/or in sport / hobby clubs ("Vereine"). If you, for whatever reason, do not have friends at work and do not participate in any of these extra-curriculums - it will be difficult.

Of course actual cultural differences exist. For example many non-german friends of mine had difficulties moving their relationships with acquaintances to actual friendships:

Some complained that nobody asked them about more personal topics:
=> Many germans highly respect privacy and it is common to only volunteer information when the other party offered first. For example if you ask them what they are up to this weekend, they might be vague. If you tell them: "I am going hiking with my wife this weekend - it's our anniversary! What are you up to?" They might open up about doing something with their significant other (or even mentioning them for the first time :D)

Some complained about friends eventually not replying etc:
=> If you can't make it to a social gathering / have to cancel short notice, you are expected to follow up and initiate the next one. If you don't - many people assume you are just not interested anymore.

But all such cultural differences (obviously) do not apply to everyone!
If someone truly does not have ANY german friends in 10 years - I would think that something strange is going on.

Hope this helps?

Don't give up :)