I-Q FIR filter using verilog/verilog-a gives no output on one channel

I built up an I-Q demodulator using verilog-a blocks. At its output,
both the I and Q paths go through an identical FIR filter. The output
of the I path looks like what I expect, but the Q path is basically
zero - it has 250 dB lower gain than the I path.

I've tried a bunch of things: I re-created the filter symbol and
verilog code from scratch in the library manager; I deleted all of the
compiled AHDL code belonging to the filters and forced it to
re-compile; I created a second version of the filter from scratch. I
get the same result no matter what. If I hook the I-channel up to the
Q-channel's filter, I get the correct output; if I hook the Q-channel
up to both filters, I get garbage out of both.

What's happening here?

 

Without seeing the example, it's probably impossible to tell.

Andrew.