Do I point to mistakes in a paper if I cannot contact the authors? I am about to submit a manuscript in which I point to some methodological errors that I found in some literature. The error may significantly change part of the results and conclusions if those papers. 

I thought I should first contact the authors of those papers to discuss the issue with them. Especially, to check if they actually made that mistake or if their methods were not detailed enough.

However, most of the authors that I tried to contact did not answer back (I checked their actual email address) or did not remember the details of their analysis.

What should I do? Am I "allowed" to point to some papers that I repute wrong only on the base of the details given in their method section?
Of course you can publish it if you think someone is wrong. This is the entire point of science. Always do so in a nice way though: it is very well possible that everything appeared to be correct with the knowledge at the time the paper was published. Or that it is you who misinterpreted the methods (possibly because the paper was a little vague), as you suggest in your question.

Even if it was really a mistake: everybody makes mistakes from time to time and that is nothing to be ashamed about. You did the right thing by contacting the authors first. Not only is it the correct thing to do, but I have also seen this lead to new collaborations that resulted in multiple papers.

If both the paper and the errors were important, it is your responsibility as a scientist to correct the errors. But if the errors are minor, the result is "unimportant", or you are not 100% sure that there actually is a mistake, you could also decide to simply ignore the paper. There are plenty of bad and/or unimportant papers out there and frankly, they do not deserve the attention even if it is to correct mistakes.