multiple imputation

I have missing values from a few subjects due to instrumentation not working. 
My data set is N=283 data points. For some subjects i have 60 data points
missing max. 

I tried to use Amelia 2  to impute the missing values but i am getting a
negative number and i am sure this is wrong because its biologically
implausible to have a negative number for what i am measuring.

Does anyone have any suggestions on how to proceed?

Thanks.

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