model coefficients are very close but incorrect! [HELP IN A PROBLEM]

[YaadR]

I'm having trouble with the model equations result. This is the best i could get and it is all but shy from perfection. The coefficients are reasonable but for some reason it clamps to zero the 'u0' coeff which suppose to be 10 and approximate 'tanh(a*x0)' to almost 10 which suppose to be clamp to zero. I've already tried many possible parameters and the below image is the best i can get, and its is not suppose to happen, any enlightenment will be appreciated

[Jacob-Stevens-Haas]

for some reason it clamps to zero the 'u0' coeff which suppose to be 10 and approximate 'tanh(a*x0)' to almost 10 which suppose to be clamp to zero

Are you saying u0 is approximately tanh(a*x0)? If so, then the model will likely be unable to disambiguate between the two.