convergence error code in mixed effects models

Dear All,
I want to analyse treatment effects with time series
data:  I measured e.g. leaf number (five replicate
plants) in relation to two soil pH - after 2,4,6,8
weeks. I used mixed effects models, but some analyses
didn?t work. It seems for me as if this is a randomly
occurring problem since sometimes the same model works
sometimes not.

An example:

names(test)

[1] "rep"    "treat"  "leaf"   "week"

library (lattice)
library (nlme)
test<-groupedData(leaf~week|rep,outer=~treat,test)
model<-lme(leaf~treat,random=~leaf|rep)

Error in lme.formula(leaf~ treat, random = ~week|rep)
: 
        nlminb problem, convergence error code = 1;
message = iteration limit reached without convergence
(9)

Has anybody an idea to solve this problem?
Thanks

Ilona

Ilona Leyer
Conservation Biology
University of Marburg
Germany
e-mail: ileyer at yahoo.de