Almost half of the teachers who took a professional headship qualification have no intention of becoming heads, according to a survey commissioned by the Assembly government.
Two-fifths of respondents who were non-headteachers but held the National Professional Qualification for Headship (NPQH) said they were unlikely to become a school leader.
Some said it was because of the negative aspects of headship, while others said there was not enough additonal pay for the extra responsibilities.
Half said their main motivation for taking the NPQH was to go on to become a head, but a third said they saw it as a good form of professional development and were not necessarily thinking about becoming a head.
The survey also found that a quarter of those who had taken School Leadership Modules (SLM) had no desire to progress to headship either, mainly because of the additional workload, stress and lack of contact with pupils.
The research, by independent consultants, found that although there has been a drop in the number of older heads in Wales, more than a third of heads are due to retire in the next five to 10 years.
Local authorities are increasingly having to readvertise vacant headship positions.
A spokesman for Anglesey Council told researchers: "Twenty-five years ago we would get between 20 and 30 applicants for a primary headship and between 10 and 15 for a secondary. Now we are lucky if we get six for a primary and two or three for a secondary. We just can't shortlist any more."
Rural areas reported having the greatest difficulties in recruiting. Carmarthenshire local education authority reported "severe problems".
A spokesman said: "Being able to shortlist for headship posts is considered to be a success in itself."
As a result of the problems, many schools across Wales are being led by acting heads - in Gwynedd, the figure is around a third.
Carmarthenshire has had to cluster several small schools under one head to solve the recruitment problem.
Headteacher unions said the findings highlighted the need for increased funding for better training opportunities.
Anna Brychan, director of the NAHT Cymru, said: "This is a critical area - not least because the flagship school effectiveness framework (SEF) relies so heavily on school leaders to drive improvement."
An Assembly government spokesman said: "The findings will be very useful as we develop proposals for a new system to better support individuals, and will also enable us to deliver the aims set out in the SEF."
Original paper headline: Nearly half of teachers qualified as heads `have no intention to lead'