I actually encountered this problem with one of my variables (perceived risks) which is hypothesized to have a negative effect on my other variables. When I ran correlation, it is not significantly correlated with the other variables. However, when I ran convergent and discriminant validity, perceived risks yielded acceptable values. I also tried running my model on SEM and the model fit is also okay despite the insignificant correlation of perceived risks with the other variables.
I'm also new to using SEM so I still need some enlightenment :) Greatly appreciate any advice (and if you could also share some valuable material about this). Thank you!