A submission has been made to the Public Audit and Post-legislative Scrutiny Committee which is currently examining the Freedom of Information (Scotland) Act 2002.
It calls for the law to include private companies delivering public contracts, with some worth as much as £7 billion, and raises concerns about the roles of special advisers and their influence in the process.
The submission also highlights critical meetings which were attended by senior SNP ministers but not minuted, including on the future of the Police Scotland Chief Constable.
Tavish Scott MSP said the act "revolutionised the public's right to access information", calling it "one of Holyrood's greatest successes".
He added: "However, I have told the Committee of a number of questionable FOI practices, such as the Scottish Government interfering in local authorities responding to freedom of information requests.
"All too often we're finding political advisers have too much power or the public interest test is being wheeled out as a get out of jail free card.
"Conducting high-profile high-interest meetings without taking notes rips up the FOI rulebook and is a tactic to avoid paper trails to keep Parliament and the public in the dark."
The party also refers to a request for information relating to outstanding work on the new Queensferry Crossing.
After being months late, the request was rejected because it could be damaging to the contractors - however the Lib Dems say Parliament eventually acquired the information "to no apparent detriment".
Mr Scott said: "As we celebrate 20 years of devolution we should turn our attention to how FOI law needs to evolve and expand, as its original architects always intended.
"That needs to start by opening up private companies with big public contracts to the scrutiny they deserve.
"The public want to know whether the contractors maintaining our roads, or running our railways and prisons, are doing a good job.
"The ScotRail contract is worth £7 billion - more than the budgets of many councils and government departments. In return, the public should have the right to ask them basic questions."
A Scottish Government spokeswoman said: "Scotland already has the most open, far-reaching freedom of information laws in the UK - we are working to widen the coverage even further and welcome all suggestions as to how that can be done.
"Legislation has previously been expanded to a range of arms-length organisations providing cultural, leisure and sporting services on behalf of local authorities, private prison contractors, providers of secure accommodation to children and young people, and grant-aided and independent special schools.
"We will continue to assess options to further expand coverage, and whether other private companies exercising public functions should be brought within scope."