question about charge pump circuit
Your first order of business in checking out the real circuit is to see if the 555 output is able to maintain a reasonably good looking square wave with full swing from near GND to up near the VCC level of 12V. If the output cannot achieve this then there is no hope to be able to step the voltage up as you wish. The output should have an aquare waveform with a 50% duty cycle at a frequency of about 45kHz.

If the output is acting up then you need to make sure that you have GND hooked up to the 555. Also check out the VCC (+12V rail). Is it staying nice and steady at the 12V level? If the +12V level is sagging way down then there will little chance of doubling the output voltage. Make sure that the battery that you are using is not some 3/4 dead unit with a high output resistance.