Hello,
I've just build a two-switch flyback converter (Vin=48VDC, Vout=12VDC, Pout=50W) but the boostrap circuit for the high-side driver doesn't work quite well. That's it, during the Mosfet switches OFF period, the low-side freewheeling diode is only forward biased for a (too) small period of time (I suppose) hence the bootstrap capacitor doesn't get enough charge.
I've seen a workaround for this issue, by adding an extra switch across the low-side freewheeling diode and make it close (ON state) during main switches OFF periods.