Look for a physical cause for your insomnia. The pills only treat the symptom.
I would not recommend taking more than one of the 10mg Ambiens. If you're finding that it's no longer helping you sleep, check with your doctor and see if there is anything else that he/she coud prescribe for you. There are other sleep meds out there that might help.
Also, have you tried the Ambien CR? It is a little stronger than the 10mg Ambien, but it is also controlled release, so it is not only supposed to get you to sleep faster, but also allow you to stay asleep. You might find that simply changing to this formula would be helpful.
Are you taking your Ambien on an empty stomach? If not, try that also - food does delay the "sleepy" effect of Ambien, so if you're taking it on a full stomach, that could be why (at least partially) that it's not working for you anymore.
Best of luck!
If you are taking ambien too constantly, then your brain simply cannot sleep without it. With it, it sleeps like normal brain without ambien, which means that it could be addiction.Try not taking it for a week and then take 10mgs and see what happen.