In Fortran types with a name starting with is are not shown in the documentation as the they are considered as a "type is".

A small example showing the problem(used doxyfile is a standard file generated by doxygen -g):
      !> \file
      !> file documenation module_tst

      !> @brief documentation module tst
      module tst
        !> @brief the TYPE IsAProblem is not shown by doxygen 1.8.5
        TYPE IsAProblem
          INTEGER :: element !< element in type isAProblem
        end type

        !> @brief the TYPE NoProblem is shown by doxygen 1.8.5
        TYPE NoProblem
          INTEGER :: element !< element in type NoProblem
        end type
      end module