Disappearing sprites

It seems that sprite and bitplane hardware are sharing the same DMA channels. DMA loads SPRxDAT registers right after horizontal blank (even sprites go first, odd later) and BPLxDAT register load sequence is started when beam-counter reaches horizontal position specified in DIWSTRT. So if that position is too near to left absolute border (the endpoint of horizontal blank), sprites disappear. It is common in amiga monitor drivers for 30kHz or higher horizontal frequency modes that there are only even sprites visible, just because these modes have left border too narrow.