Primary schools in England are finding it more difficult to recruit head teachers than at any time since 2000, analysis suggests.
The Education Data Surveys' analysis suggests one in four primary headships advertised in January 2013 were not filled within 60 days.
This compares to 15% in January 2012 - the peak month for recruitment.
The Department for Education says it is aware that vacancies will rise as the baby-boomer generation retires.
The figures, published in the Times Educational Supplement, show that of the 261 English primary schools which advertised for a head teacher in January 2013, 26% were forced to re-advertise within 60 days.
The previous January just 15% of the 249 job adverts were re-advertised.
The news comes as head teachers are meeting in Birmingham for the annual conference of the National Association of Head Teachers, which represents 85% of primary heads in England and Wales.
General Secretary Russell Hobby said the cause of recruitment problem was a combination of the negative "rhetoric that the profession was subject to, the ever-rising targets and the fear of Ofsted is a huge trigger for this".
He added: "Particularly in the most challenging schools people are really concerned, they are saying to themselves 'can I take the risk?'"
The situation was not being caused by people retiring from the profession but by "people not wanting to take up the job in the first place", he said.
Headship recruitment was most difficult in London where 44% of jobs were re-advertised. This was more than double the 20% re-advertisement rate of 2012.
A spokeswoman for the Department for Education said: "Overall headship vacancies are low and stable, but we have always been aware that as the baby-boomer generation started to retire we were likely to see a rise in the number of vacancies.
"Where governing bodies plan well in advance for a departure, they are much more likely to make an appropriate appointment.
"The growing network of teaching schools is also helping to develop the next generation of great heads, as they identify teachers with potential for headship right at the start of their careers and nurture them on the job over time."
Mr Hobby said that many good deputy heads were reluctant to go for the top job because of the relentless pressure from, among other things, Ofsted.
His association is due to launch a potential alternative to Ofsted, dubbed "Instead", at its conference later on Friday.
Under a pilot scheme due to begin next autumn, heads will be invited to inspect each other's schools, drawing on their knowledge of school management and leadership in a new kind of peer-review system. It will be subject to an independent evaluation in the hope it would be considered by a future administration.
Delegates are due to debate Ofsted at the conference and they will call for school improvement brought about by support and encouragement.
Mr Hobby said: "Schools dance to Ofsted's tune but don't really learn from the experience - they are too busy defending themselves against it and then recovering.
"Their leaders are passionate about delivering the best for their pupils and understand the role external scrutiny plays in providing a first-class education.
"Through Instead, heads and senior management will be offered a chance to take ownership of standards by inviting staff from other schools to challenge their judgments and plans. We have every reason to believe peer review will be challenging and rigorous, as often it requires professionals to spot problems others may miss."