Thanks Ben. Sorry my package is not yet there in github!
But I think I solved the problem by running just rstan_config() instead of use_rstan() before re-building the package. The latter command gives the error message for existing packages with stan functionality.
I guess one line should be added to the page https://cran.r-project.org/web/packages/rstantools/vignettes/minimal-rstan-package.html
In the advanced options section, it should say that everytime a stan file is changed the rstan_config() must be first issued before re-compiling the package. Otherwise, the older versions of stan files will be compiled.
Related to this, it should also say that if the stan files do not need to be recompiled then a command like devtools::install(’.’, quick=TRUE) should be issued to re-build the package.
Many thanks and best regards, Sujit