Mixed effects models

Hi,

I am currently trying the exercises of Crawley on mixed effects model (ex. 7). I remade the example on page 33/34 and am currently trying tho do this analysis with my own data. I have a response variable (size=MiWi), measured at different dates (date) on the same individual (obs_nr = random variable). Further more I have insects species (ins_spec) and host species (host_spec) as fixed variables. When I run it I get the error message:

cocon_mixed<-lme(fixed=MiWi~host_spec*ins_spec,data=cocon_nash,random=~date|obs_nr) Error in lme.formula(fixed = MiWi ~ host_spec * ins_spec, data = cocon_nash, :
nlminb problem, convergence error code = 1
message = iteration limit reached without convergence (9)

What is going wrong?

Kim