Only one in six Welsh schools had to readvertise headteacher vacancies last year - compared with one in three for England and Wales as a whole. In inner London, nearly three in five schools failed to appoint a head at the first attempt.
Professor John Howson said the overall findings from his annual survey of headteacher job adverts were the worst since his research company, Education Data Systems, began the study 21 years ago.
More than 2,600 schools in England and Wales needed to advertise head posts last year, of which at least 1,000 needed to place adverts for the same post again later.
The proportion of posts being readvertised increased between 2004-5 from 27 to 36 per cent in secondary schools, and 37 to 38 per cent in primary schools.
Faith schools and those in the capital were, as in previous years, the worst affected. The proportion readvertising jobs was 59 per cent for inner London, 44 per cent for Church of England and 59 per cent for Roman Catholic schools.
But in Wales, "there are relatively few difficulties recruiting to the leadership group", said Professor Howson.
He added: "Schools that don't have a permanent head will have problems developing their policies, their parents may drift to a more settled school, and their pupils' education will be affected."
Professor Howson said reasons for schools' difficulties included the boom in staff reaching retirement age, which is expected to peak in 2008.
In Wales, a third of the 1,833 heads in post last year were aged 55 or over, and are due to retire over the next five years. But the number of primary schools has declined by 43 since 2000-01, and more - including some secondaries - may close as pupil numbers continue to fall.
Secondary head salaries of more than pound;100,000 are becoming increasingly common, even outside London, and primary heads' pay is also beginning to catch up, according to Professor Howson.
But he said the gap between pay for experienced teachers and members of school leadership teams remained too narrow, giving little incentive for staff to seek promotion.
Teachers' pay and conditions in Wales have not been devolved to the National Assembly.