Payment Failed – No Redirects to the payment site

We have an issue (already had this since a few months), that a lot of payments fail.
The reason is that a lot of those do not get redirected to the payment page.

Is this a known issue?
Does anyone else have this issue?
We need help to resolve this, since almost every 2nd payment fails (for our tests it works almost flawlessly but for real customer it really doesn’t.

Thanks for any helpfull input!

Kind Regards