It's mostly a matter of time. Eventually, if the mycellium survives, it will produce more. Just wait, you'll see. Try letting the CO2 level build up in your fruiting chamber for 1 (one) day and see what happens. I believe I read somewhere that carbon dioxide is supposed to play some part in initial pinhead formation. I'm not sure on that, let me go look it up..
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...According to The Mushroom Cultivator, pre-pinning occurs with a CO2 level between 5000 & 10000 ppm (parts per million), while primordia formation occurs below 5000 ppm. I have no idea what the average co2 level is for breathable air, so it's your call.