The couple of hints I have for g-spot location are
1) get her in the mood, excited, aroused with clitoral stimulation and all the usual suspects FIRST. Many times if the woman isn't aroused you can touch the g-spot and it's uncomfortable. The g-spot (as well as the clitoris) is made of erectile tissue, and so gets bigger when aroused as blood flows into it, so it can be easier to find then as well.
2) You can find toys with an "upcurve" that can make the exploration a bit easier. Also try other motions than just the up and down version (like the come hither with the fingers does), also try side to side and circles as it can feel different.
3) Sometimes it can be easier for the woman to do some exploration on her own, as she can move the toy into places that feel good easier than trying to direct from above!
4) And the biggest thing is to just have fun with it. Whether or not you figure out what works, it's still a lot of fun, and any stress or "worry" about it can seriously affect any chance of success. (That goes with sex in general).
I know many women who thought they had NO g-spot at all, that have since had g-spot orgasms with some trial and more trial! Many women find as they age, their sexual response changes and sometimes this can make everything feel a bit "different'.
And in the end, g-spot orgasms aren't the end all be all, but just one of many ways to feel sexual.