Lock explained that the most high-value contracts are not necessarily good for business, as they may not reap the biggest profits.
He was speaking at the APL and Society of Garden Designers (SGD) joint autumn workshop on 17 September at Capel Manor College.
The issues of staffing, weather, client satisfaction and paperwork were explored during his seminar on contracts at the event.
"There is always something that will potentially be a problem," warned Lock. "When you go into a site assess everything: check for access issues, underground and overground cables, water and the possibility of deep excavations.
'If the contractor and designer haven't done their homework properly they'll end up with problems."
Issues that arise unexpectedly can mean the pricing of the contract is incorrect, leaving the contractor out of pocket and the client unhappy, added Lock.
One of the key actions that could help avert problems that arise is ensuring all paperwork is kept up-to-date and filed in an orderly way.
"Working in a chaotic environment means a chaotic mind," he said. "It is essential to make sure the discipline flows through from the office onto the site."
Lock also urged contractors to share their experiences on professional networking website LinkedIn, where he has set up a landscape and garden design forum.