Hi listers

I am using linear mixed model for my analysis, my model is?m1<-lme(yij~X1i+Tti+X1i*Tti+(1+Tti),data1)? with random slope and random intercept. if i run this model it works fine but when i run it in loop or i specify correlation structure i.e.m<-lme(yij~X1i+Tti+X1i*Tti+(1+Tti),correlation=corAR1(), data1) then it give me error?Error in lme.formula(yij ~ X1i + Tti + X1i * Tti + (1 + Tti), correlation = corAR1(),? :?? nlminb problem, convergence error code = 1? message = iteration limit reached without convergence (10)so whether there is problem in model specification or in the correlation specification??