Right skewness in bootstrap distribution

Dear NONMEM users, I am modeling some PK data using a linear 3-compartment model, in which drug concentrations were measured in two of these compartments simultaneously after i.v. dose. The model fits the data reasonably well, and all parameters seem reasonable except for V1 (volume of the central compartment, which occurs to be the dosing compartment). Estimate for V1 is very small, what does not make sense considering the average dose given and the mean Cp0 calculated by NCA. This result suggests drug distribution is restricted to plasma, however it was observed extensive distribution to tissues. IIV for V1 is relatively small (19.6%, n=8 subjects). The histogram for V1 (nonparametric bootstrap with 100 replicates) shows a right skewed distribution with the presence of a subpopulation and broad confidence interval (5th percentile tends to zero). I tried to solve this by fixing V1 to a reasonable value, running the model to calculate all other parameters, and then changing the initial estimates to these parameters in order to recalculate V1, but it turns out to the same small estimate. Any suggestions will be appreciated! Thanks in advance. Felipe