
I am doing a fit for the decay time of particle to obtain the lifetime.

For the purpose of my question, consider I only have the signal component.

so my lifetime pdf is , f(t,dt) = f(dt) X f(tdt)
f(tdt) is convolution of exponential with a gaussian resolution function using event level errors in decay time.
f(dt) is the pdf for error dt in decay time t.
All the fits are the unbinned maximum likelihood fits
 If I fit for just f(tdt), fit results are good, decent chisquare/ndf, MIGRAD converged and MINOS gives an accurate error. attached data set and fit macro
d_lifetime1.C (4.8 KB)
dsm_new.txt (864.7 KB)
 If I fit for just f(dt) , fit results are good, MIGRAD converged and MINOS gives an accurate error. Attached are dataset and fit macro.
t_error.C (4.9 KB)
signal_new.txt (451.2 KB)

But when I use RooProdPdf sig_3d(“sig_3d”,“sig_3d”,f(dt),Conditional(f(t),t));
see attached dataset and fit macro
ds_lifetime.C (6.2 KB)
dsm_new.txt (864.7 KB) 
The fit converge well with decent chisquare/ndf and residuals but I got lots of following warning

I don’t understand the reason for the above warning, since I didn’t get any such warning when I tried the fit for t and dt separately.

I made sure all the initial values, lower and upper limits for shape parameters are same as when used for fitting t and dt separately

I also tried fixing the shape parameters for dt, but again I get the same problem.

I understand that my results with final pdf even with warning messages is correct since Migrad converged well,

Intermediate warnings might come from fitter trying to scan over wrong parameter space?
If any RooFit expert can comment on the possible reason for the above warning message and how to avoid them ?
Thanks