About ISR

Dear SIr,
When I did the simulation about ISR at the e+e- colliders with the new physics model. I found that there would be some mistakes.

When I do the ISR simulation of the process e+ e- > a a a NP=2 QED=2 with the new model， everything is going well. Pythia works well, too.

However，when I do the ISR simulation of another process e- e+ > b b~ ve ve~ NP=2 QED=2 $$z with the new model， the program reports an error:

Survey return zero cross section.
   Typical reasons are the following:
   1) A massive s-channel particle has a width set to zero.
   2) The pdf are zero for at least one of the initial state particles
      or you are using maxjetflavor=4 for initial state b:s.
   3) The cuts are too strong.
   Please check/correct your param_card and/or your run_card.
Zero result detected: See https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/FAQ-General-14

The run card I set is as follows:

set lpp1 -3
set lpp2 3
set ebeam1 500
set ebeam2 500
set polbeam1 -80
set polbeam2 20
set pdlabel isronlyll
set nevents 10000
set ptb 5
set etab 5
set drbb 0.4

I don't know why this happened. I would appreciate it if you could answer it for me. Thank you!