Is every e-resource has a DOI and why sometimes I can’t find DOI?

Not every article has a DOI.

You might be unable to find DOI for the following reasons:

• DOI is a new thing from 1998. So some age-old documents don’t have DOIs like articles published prior to 2000.
• There are some presses or journals haven’t registered a DOI. Thus, they don’t require authors to tag the DOI.
• The article may be too new to enter DOI system.