Help! SAS NLMIXED. The final Hessian matrix is not positive definite?

Hi, there,
I am currently running sas PROC NLMIXED to analyze my data. My sample are 27
animals. Their running speed were reperatedly measured at 9 temperature
treatments. The running speed is tested whether it can be predicted by
temperatues using a logistic-expotential function. Therefore, the function I
wrote in SAS code is an logistic-expotential.
My sas code is listed as following text(A). The problem I faced is that there
is a warning in log page, which is as following text(B). Could anyone give me
any suggestion to patch it on? any suggestions would be highly appreciated.
many many thanks.

Shu-Ping

text(A), My sas code:
proc nlmixed data= sprint_30s method=firo ;
parms b1=0.04 b6=4.5 b2=0.2 b4=0.2 U1=0 U2=0 VAR=0.1 t11=0.1 t22=0 t12=0;
bounds VAR>=0;
LOW=1/(0.015+U1+b6*exp(-b2*(Temp-2))); HIGH=1-exp(b4*(Temp-43.1)+U2);
pred=b1*LOW*HIGH;
VAR1=t11*t11;
cov12=t11*t12;
VAR2=t12*t12+t22*t22;
model speed_m~normal (pred,VAR);
random U1 U2 ~normal ([0,0],[VAR1, cov12,VAR2]) subject=ID;
run;

text B:
WARNING: The final Hessian matrix is not positive definite, and therefore the
estimated
covariance matrix is not full rank and may be unreliable. The
variance of some
parameter estimates is zero or some parameters are linearly related
to other
parameters.

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