Most women don't orgasm without clitoral stimulation. It is possible, but it usually takes practice. The g-spot can be tricky to find, especially if you're not very aroused. Make sure you spend plenty of time on foreplay. The more aroused you are, the easier it is to find the g-spot. When you do find it, try applying a lot of pressure to it.

If you want to orgasm more easily, try to take your mind off it. If you're thinking about reaching orgasm and wondering how long it will take, it will make it more difficult. Try to just enjoy yourself without concentrating on the end result. Good luck :)

You might need to see a sex therapist on this one.
Part could be psychological or emotional.
Some women have problems reaching orgasms.
Try different methods to get more aroused before sex and more foreplay. Make sure your really aroused and wet before he pentrates.