I don't know exactly how does it work, but you are using positive feedback to get more gain, aren't you? Because it seems to a regenerative circuit. And maximum positive feedback limits the loop gain before oscillation.
So you should just increase the positive feedback to get more gain until the oscillation, and if it is not possible, you should rather increase the sensitivity by decreasing the noise figure of the preamp.
Choose a transistor which has got lower noise figure maybe, or increase the current consumption of it.