This emerges from a newly published report from the education inspectorate on how the curriculum is delivered in FE.
It comes in the wake of another report from HMIE and the Scottish Further Education Funding Council which delivers an upbeat verdict on the performance of colleges, now that all 46 have undergone inspection - although it has strong criticism for the quality of some teaching.
The curriculum report reveals flaws in tracking students in the system, and calls for more "robust and accurate" data. While statistics are more available for full-time and day-release learners, this is not so for flexible learners because of their irregular patterns of study.
The report states: "It was not always easy for colleges to determine whether learners were still active on a programme if it did not involve attendance at college or the submission of assessable work.
"As a result, colleges did not always know which of their flexible learners were still actively studying. A large number of colleges only withdrew students at the end of the academic year and re-enrolled them at the beginning of the next if they knew that the learners were still actively studying. Otherwise, the implicit continuation of learners' involvement was equated to retention.
"Consequently, retention rates, generally very high and frequently reported at 100 per cent, were unreliable."
The report also goes on to identify flexible learning as a problem in the accuracy of student attainment figures. Only a few colleges were able to quantify attainment for students taking Scottish Vocational Qualifications, with descriptions from other colleges ranging from "high" to "patchy".
These variations were due to the lack of an agreed method of calculating the number of students satisfying attendance criteria because of flexible starting dates and course duration.
Inspectors also found "wide fluctuations" in attainment on certificated courses involving flexible study - one college reported an open learning attainment of 8 per cent; others reported 65 per cent.
HMIE encountered the same suspect results for student achievement on non-certificated flexible programmes, as was the case for retention figures. They were frequently close to 100 per cent and the programmes "had no criteria for success other than completion of the work, which was largely self-reported".
The information was therefore "unreliable in many instances".
Inspectors commended colleges that delivered learning for such students through virtual learning environments (VLEs), which meant they could monitor logins to the system and track the activities of learners online.
The report recommends that colleges check out how and what students have learnt previously before directing them to a particular mode of flexible learning.
HMIE also suggests that the funding council should devise methods of calculating student retention and attainment data for flexible programmes.
Inspectors, meanwhile, have been scrutinising the effectiveness of staff development and concluded that overall this was "robust and effective". A substantial number of colleges had well-developed arrangements which they were fully implementing, while "the remainder had at least adequate arrangements and were at various stages of improving them".
But, while colleges were effective in dealing with national and college priorities, a majority did not deal with career and future skills development systematically enough, according to the report. Awareness of the importance of this was, however, increasing.
Another weakness concerned the need to link staff development to college performance, through monitoring the impact which improvements in the quality of teaching have on student results.
There must be "more explicit links between staff development and quality improvement".