Why is it that when the dentist uses the regular lidocaine it does not work for me but the septocaine does?? what is the difference between the two?

Molecule Size: Although the below referenced article may be too technical to be of any use, in summary the smaller molecular size of the active ingredient in Septocaine may allow for better absorption into the bone for local infiltration. You may have really dense bone in the areas that needed treatment. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3589513/

Additives, structure: Septocaine is a local anesthetic (articaine) mixed with epinephrine to stop bleeding. Lidocaine & Articaine are "cousins" - both anesthetics, but different structure (arti has an ester & thiophene ring, lidocaine has benzene ring). Supposedly, articaine works better on bone than lidocaine. Some people hydrolyze esters slower than others. They both can cause side effects, though. Use what works.