Two-thirds of councils are failing to get many offenders into work placements quickly enough to meet official guidance.
However, the Scottish Government says there have been "significant improvements", with the system now working more quickly than ever.
Tough community service orders were at the heart of justice reforms earlier this year effectively to phase out short jail sentences of three months or less for minor offenders.
But in seven of the country's 32 local authorities, fewer than half of community service orders begin within the government's target of one week, according to information obtained for September of this year by the Liberal Democrats under freedom of information.
Only four areas saw all offenders begin work within a week of sentencing.
Liberal Democrat justice spokesman Robert Brown said: "This is not at all a satisfactory position. Liberal Democrats have always been clear that community sentences have to be speedy, effective and of high quality.
"Robust and effective community sentences are a much better option than very short prison sentences, which just don't work.
"But in order for community sentences to fulfil their real potential for both offenders and communities, these work placements must be properly implemented, within given time frames."
Argyll and Bute saw just 15.4 per cent of the 13 community service orders handed out in September being started within seven days, while just 12 per cent of the 17 orders in West Lothian were started on time.
Fewer than a third of the 41 offenders in Aberdeen who received community service orders started them within seven days, while almost a fifth took between two and three weeks to get started.
Mr Brown added: "In some areas, offenders kick their heels for almost a month before their sentence begins. This does neither the victim nor the offender any favours."
The Scottish Government issued guidance in 2009 stating all community work placements should start within seven days.
The Criminal Justice and Licensing Act, passed earlier this year, included a "presumption against" sentences of three months or less, with ministers pledging that tough new community service would be introduced for minor offenders.A Scottish Government spokesman said: "What statistics show is that offenders are now being sent out to repay their debt to the community more quickly than ever before, with the majority starting their manual labour within seven working days, and finishing within 12 months - and we have brought in legislation to ensure that in future this is the case for all offenders in all authority areas.
"There have been significant improvements right across the country from 2008 and follow on from the swift action this government took to help support those on the ground, helping them to improve their procedures, whilst increasing funding to record levels to strengthen the system and speed up start and completion times.
"The justice secretary has made clear he will be looking for community justice authorities to build on the performance."