[R-sig-ME] problems with the variance in lmer

[catferr]

Dear all
I am using lmer to analyze data on repeated measures of  physiological and hormonal parameters. In two cases (on five parameters that I have) the variance of the random term is equal to zero ( 0.000), suggesting an error in the way I am modelling the data. I was wondering if there is any function to define different correlation structure with lmer (I did not find a lot about it) or if you have other cues that I could try .
Thanks a lot in advance 
Caterina Ferrari
UQAM

Here the output of one of my model

modid<-lmer(increase~I.II+sex+age+(1|ID), data=cort)
>summary(modid)
Linear mixed model fit by REML 
Formula: increase ~ I.II + sex + age + (1 | ID) 
   Data: cort 
   AIC   BIC logLik deviance REMLdev
 249.1 258.9 -118.6    243.2   237.1
Random effects:
 Groups   Name        Variance Std.Dev.
 ID       (Intercept)  0.000   0.0000  
 Residual             39.419   6.2785  
Number of obs: 38, groups: ID, 18

Fixed effects:
            Estimate Std. Error t value
(Intercept)  2.10456    3.59425   0.586
I.II         0.07807    0.12102   0.645
sexm        -1.56851    2.29559  -0.683
agesubad     0.61569    2.26940   0.271

Correlation of Fixed Effects:
         (Intr) I.II   sexm  
I.II     -0.838              
sexm     -0.167 -0.292       
agesubad -0.315  0.128  0.025