The Lobbying (Scotland) Act 2016 requires face-to-face meetings between ministers or advisers and outside groups to be registered by those who seek to lobby.
But calls made over the phone, or those instigated by ministers, do not have to be registered.
Lobbying done via email or messaging apps such as WhatsApp are also not covered by the Act.
Analysis by the Herald and investigative outlet the Ferret of ministerial engagements showed “hundreds” of meetings which were undertaken but not subsequently reported to the register, the newspaper said on Monday.
Scottish Labour business manager Neil Bibby said the legislation should be “looked at”.
“These reports raise serious questions about whether Scotland’s lobbying laws are fit for purpose.
“With countless key meetings going unrecorded, it is clear that the SNP are exploiting loopholes to avoid scrutiny.
“These shameless attempts to undermine the spirit of the lobbying act are all too typical from a government constantly avoiding any and all transparency.
“The laws need to be looked at, but legislation can only do so much as long as there is this culture of sleaze and secrecy at the heart of the SNP.”
The Scottish Government said it was “committed to transparency”, adding that “all ministerial engagements are recorded and published, including meetings held via phone calls”.