The middle register (mud register) of the horn is generally last to develop and requires dedicated practice. As part of his Invested Musician Community, Andrew Bain, developed an exercise to help horn players who have difficulty in this range - me being one of them. Horn players must be agile in all registers of the instrument. Long gone are the days when players could specialize as "high" or "low" horn players. Do not believe the mumbo jumbo out there saying some people are "genetically unable" to flutter tongue. Some simply have a greater proclivity than others. Practice it and it will happen.