Not all pesticides work with LC and not all work with GC.
Compare this list that work well with LCMSMS
https://www.restek.com/en/products/refe ... ure/40244/To this list that work well with GCMSMS
https://www.restek.com/en/products/refe ... ure/41952/I notice that 2,4-D is not in either of those lists, but I have used LCMSMS to analyze for it before, but the sensitivity is less for it than most other analytes.
The ones you have trouble seeing my require different polarity of the source, or different temperature and flow rates versus the rest of your list. This can be a problem when trying to do a large list in one injection. You cannot optimize for all the analytes at the same time, so some will have good sensitivity and others not so good. May need APCI and not ESI. Not sure of your instrument setup to know what options you have.