Why several of isomers of octane such as 2,3-dimethylhexane and 2,4-dimethylhexane doesn't have a value for melting point from any databases? This is a very rough data (as shown below) I have collected from CRC Handbook of Chemistry and Physics 93rd edition in black and ChemSpider in blue. As you can see, I am not quite done, but I was wondering why several of the isomers of octane such as 2,3-dimethylhexane and 2,4-dimethylhexane doesn't have a value for melting point from both databases? (I have tried searching up the melting point for each on a reliable database and have come up with no value given. Wikipedia does give a melting point, but I am very cautious about using that data as it does not a source to prove it.
I recommend to include more than one (secondary) reference and to widen the querry -- if there is access -- to general chemistry data bases like Chemical Abstracts (by the American Chemical Society), or Reaxys (by Elsevier), and Springer Materials, too.

In addition, do not stick to one chemical name, and equally include in your querry unique identifiers like the CAS registry number -- applicable regardless how the organic nomenclature is applied (including databases in other languages) -- rather than one chemical name.  By doing so, you may rapidly find addtional data, for example among chemical suppliers like Sigma-Aldrich, that lists 2,3,4-trimethylpentane (CAS-RN 565-75-3) in its catalog with a melting point of −110 °C and a boiling point of 113-114 °C.

Beside electronic ressources, consider additional ones available in print, like volumes in Landolt-Bornstein dedicated to condensed matter (now included in Springer Materials).