R help - Nov 2013 - Loglogistic 4-parameter model, fitting multiple curves specific estimates give NaNS, when fit alone all parameters estimated

Hello,
I am fitting curves in drc to a Log-logistic 4-parameter model.

My experiment is looking at fall and spring applications of a herbicide on
wild oat at 5 different locations.  A preliminary mixed model ANOVA showed
that timing was significant.  I am running a dose response curve for each
application timing at each location.  However, when I run them all
together, my one fall location has estimated b,c,d and e, however the
standard error for all of them is NaNS.  When I run only the fall location
on its own, there is no problem in getting standard errors for all my
parameters.

What is the difference between running it alone and running it with other
curves?  How can I fix the NaNS standard errors in the multiple curves as I
need to be able to compare ED50s.
The data for the curve alone is below.  When I run it with the rest of the
curves they are separated by biotype.  Dependent variable is square root
biomass (srbiomass) and treatment (Trt) is the independent variable.

 Location Timing biotypeno Trt Rep Trtno biomass srbiomass  Kinsella Fall 5
0 1 1 21.2 4.604346  Kinsella Fall 5 0 2 1 168.72 12.98923  Kinsella Fall 5
0 3 1 86.76 9.314505  Kinsella Fall 5 0 4 1 14.3 3.781534  Kinsella Fall 5
50 1 2 12.7 3.563706  Kinsella Fall 5 50 2 2 46.44 6.81469  Kinsella Fall 5
50 3 2 9.84 3.136877  Kinsella Fall 5 50 4 2 32.38 5.690343  Kinsella Fall 5
100 1 3 10.3 3.209361  Kinsella Fall 5 100 2 3 41.84 6.468385  Kinsella Fall
5 100 3 3 0 0  Kinsella Fall 5 100 4 3 4.32 2.078461  Kinsella Fall 5 150 1
4 3.84 1.959592  Kinsella Fall 5 150 2 4 9.34 3.056141  Kinsella Fall 5 150
3 4 25.9 5.089204  Kinsella Fall 5 150 4 4 1.46 1.208305  Kinsella Fall 5
200 1 5 6.6 2.569047  Kinsella Fall 5 200 2 5 0 0  Kinsella Fall 5 200 3 5
72.94 8.540492  Kinsella Fall 5 200 4 5 24.56 4.955805  Kinsella Fall 5 300
1 6 6.36 2.521904  Kinsella Fall 5 300 2 6 3.6 1.897367  Kinsella Fall 5 300
3 6 31.28 5.592853  Kinsella Fall 5 300 4 6 1.46 1.208305  Kinsella Fall 5
400 1 7 2.02 1.421267  Kinsella Fall 5 400 2 7 11.2 3.34664  Kinsella Fall 5
400 3 7 3.02 1.737815  Kinsella Fall 5 400 4 7 0 0

Thanks in advance for any help.
Breanne

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