WDF continuous reader for interrupt endpoint.

We have a WDF driver using continuous reader for an interrupt endpoint. The maximum packet size was set to 8 bytes in its endpoint descriptor and device always send 8 bytes to the host. We set the continuous reader’s transfer length to 8 as well, however, if device sends two 8 byte packets, our callback (completion) routine was only being called once and until modem send the third 8 byte packet, and then host receives the second one. The question is why the framework holds the packet? I know probably we can change the endpoint descriptor maximum packet size to workaround this issue or send zero length packet from the device, but is there any way we can do it in driver avoid above change?