You've got four options when you just can't get your employees to do something you've asked of them... and some of these options are better than others. Read HR Guy's take on how to proceed.
When your employees question your policies or act in unexpected ways, it's best to ask "Why?" before simply reacting or saying "Because I said so." This post explains the reason.
To be blunt: your technicians NEED to appropriately track their hours, no matter what type of compensation plan they're on. And your brand (and possibly your bottom line!) is at risk if they don't. This is one rule you need to strictly enforce...
What you allow one employee to do, expect everyone to do. Here are a few tips for setting and maintaining expectations so one "bad apple" won't spread rot across your entire Brand.

We’re not a generic HR firm. We’re built for service companies with crews, callouts, overtime challenges, and fast-moving schedules. You get practical HR support that fits the real world, not corporate templates.
The HR Guy – HR Support for Service Businesses