problem in fatty acids composition determination by gas chromatograghy(GC)

Hi, guys
Recently I detect the fatty acids profile of some mutant strains. We use an Agilent GC7890 series equipped with a 30X0.25mm SP-2380 column(Supeleo),and the program is totally the same as that described in Jennifer L. Watts’ paper(PNAs, 2002). In the begining days, we got the GC traces of the N2 worms similarly as that the paper described. But in recent days we find that the fatty acids 17iso and 16:1n7 cannot be well seperated from each other. So what’s wrong? Please give me some suggestions