I used to have exactly that video converter... what setting are you using? I would try the "13 inch 640x480" setting, not any of the multimode ones. I don't think the IIsi supports any higher than 640x480 (see Here. The IIsi can use *at most* 320k of onboard RAM for video. ) and it *may* predate the Macs that supported "normal" VGA monitors.

Fair warnings, LCD monitors may not care for the "13 inch" mode, but most CRTs should at least pretend to work.