Yeah I remember the tray-loading iMacs and Powerbook G3 Series 233-300MHz had troubles booting from non-pressed e.g. burned boot CDs. My guess was because, at the time, the pressed boot CDs that shipped with all Macs were 650MB, but we were using the 700MB capacity discs.

Either that or they just didn't "like" the brands of blank CD that we were using.