The Conservatives fear the incentives may not be hitting their intended target, or that other problems are preventing disadvantaged young people from accessing the grants.
Both Assembly learning grants (ALGs) and education maintenance allowances (EMAs) are means tested. FE and university students can apply for the former, which are worth up to pound;1,500. EMAs worth pound;10, pound;20 or Pounds 30 a week are paid to eligible students aged 16 to 18.
Students can also collect pound;100 bonuses in January, July and September if they meet targets, attend regularly, and enrol for a second year of study.
The total number of FE students receiving ALG has remained steady at 2 per cent of all students for the past three years (to 20045). But take-up among university students has gone down from 17 to 12 per cent.
Figures for learners receiving EMA in 20045 show that 83 per cent collected their January bonus, falling to 75 per cent in July. But only half (52 per cent) collected the September bonus, which rewards students returning for a second year of study.
The bonus figures were due to be updated as TES Cymru went to press.
William Graham, Conservative education spokesperson, raised the issue in a plenary debate on student funding last week, noting the no one seemed to be recording whether students in receipt of ALG or EMA complete their courses.
He called for regular reviews of the grants' effectiveness, adding: "We share the desire to extend social inclusion to every level of education.
"Therefore, it gives us great concern that the latest figures indicate that, far from extending social inclusion opportunities, they appear to be restricting them."
Conservative colleague Mark Isherwood, AM for north Wales, said he had been advised by youth workers that some students were having difficulties accessing EMA funding, or struggling with delays in receiving the cash.
He added: "The EMA figures seem to show a progressive reduction in payment of bonuses, which could suggest these young people are dropping out of their courses.
"We all concur with the goals of the scheme, but if public money is being invested there should be a proper plan, and measures in place to review and adapt the scheme to ensure the goals are achieved."
Some FE students only sign up for one-year courses, so would not be expected to collect their September bonus.
An Assembly government spokesperson said: "The current academic year, 20067, is the final year of a three-year roll-out of the EMA Wales scheme.
"Until it has been fully rolled out we cannot make a full assessment of the effect it has had.
"However, the indications from the first two years are that the scheme is helping to increase participation and reducing the drop-out rate."
She added: "The bonus returns for January and July 2005 were encouraging and the slight drop in July could possibly reflect the fact that some students would have completed shorter courses and so would not receive a bonus.
"Similarly, the September bonus is paid to EMA recipients who return for a second year."