What should I do if I cannot reproduce experimental published results
@posdef's comment is already very good.

I would definitely suggest contacting the authors and trying to get exact agreement also on the details that they didn't describe in the original paper. The original authors should be quite interested in an independent replication, so they should have an incentive to collaborate. Depending on how much they contribute, they could also hope for co-authorship on your paper.

Conversely, if they are not cooperative, that is another piece of information that should be included tactfully (!) in the paper you are putting together.

If you are lucky in your analyses, you may be able to identify the exact circumstance or unreported detail that makes the difference between reproducibility and non-reproducibility, and there you have the nucleus of an original paper all by itself.

On the other hand, if you cannot reproduce the results even with help from the original authors, this should be worth at least a correspondence. Reproductions are important, and there are far too few of them!

I part ways with @posdef's comment in that I would definitely not send a letter to the editor voicing concerns about reports and reproductions not matching. This already points towards a suspicion of scientific wrongdoing on the part of the original authors. And unless you have a lot more to go by, a simple inability to reproduce a finding should not be taken as the result of foul play. There are so many potential reasons for non-reproducibility, from the ones you mention to simple random error (aka, uncontrolled conditions), that there is really no need to even imply dishonesty. Such an implication can mess up either the original authors' or your own reputation in the field for decades, so it should not be made lightly.