Mixed loads of food products have emerged as the major problem for trade across the new Irish Sea border.
"Groupage" is a type of haulage where goods from different companies for different customers are grouped together on one lorry.
It is causing problems with food products which face strict EU rules when moving from GB-NI.
One NI business has had nine haulage firms refuse to collect groupage loads of meat from GB suppliers.
A UK government spokesperson said goods were "flowing effectively and in normal volumes between Great Britain and Northern Ireland and no disruption has been reported at Northern Ireland ports".
Food products such as meat, milk and fish now need to be certified by vets when moving from GB to NI.
This certification is being phased in for retailers but already applies in full for food service and catering firms.
When the certificates are issued the lorry trailer is supposed to be sealed.
That is relatively straightforward when one company in GB is sending a load directly to another company in NI.
However, a groupage movement could involve picking up goods from several different warehouses with loads potentially moving from one lorry to another along the way.
This means trailer seals having to be broken, goods recertified and seals reapplied multiple times.
This is proving complex, time consuming and prone to error.
Therefore many hauliers are now refusing to transport groupage food loads.
This is a particular concern for small and medium food companies in Northern Ireland which regularly receive products as parts of groupage shipments.
Seamus Leheny from Logistics UK said "there are some signs from some businesses that they are getting on top of things", but he thought "the original problems from day one are persisting and for some operators they have deteriorated".
"That's around groupage, so it is picking up small consignments of food and that is something government is looking at urgently."
In a statement a spokesperson for the UK government said: "The grace periods for businesses moving goods between GB and NI are in operation and working well.
"The Trader Support Service provides free advice and support to businesses of all sizes and since the 1st January, over 99% of TSS processed declarations have been completed within 15 minutes.
"We recognise some challenges faced by haulage, such as on the issue of groupage, and we are working intensively with industry to resolve these."