You may not be doing anything wrong. Trigger points (TrPs) are hypersensitive spots in the muscle caused by overuse. They can be very persistent. If you treat the TrP but don't change your daily habits, then the aggravating factors are still present.
Most of the time, TrPs signal an active process of biochemical changes in the muscle tissue. Recent research from the National Institutes of Health (NIH) has reported on this phenomenon. Scientists were able to take tiny samples of muscle tissue without damaging the muscle or irritating the TrPs.
An analysis of the chemicals in and around the TrP showed elevated levels of many inflammatory markers. Similar testing was done on muscles in other parts of the body that didn't have active TrPs. It turns out that these same chemicals are present with just everyday regular use of the muscles but at much lower levels.
With this new information, research can move ahead in finding more effective ways to treat TrPs. For now, you may want to consider some other methods of treatment that have helped other patients. In addition to the stretching and postural changes you are already doing, acupuncture or steroid injections may be beneficial.
Talk to your doctor about what might be the next step for you. Find ways to break up your day at work. Even 10 second breaks every 10 minutes or a one-minute break every hour would be helpful. Changing positions, stretching, and practicing deep breathing are all very useful tools in a work setting such as yours.