For low firing times there may not be enough holding current available to keep the device on. This is a problem with larger SCr/triac devices and/or small loads. The other problem with the hard firing technique (keeping the trigger on for the required firing time) is you need to remove the trigger some time before the zero crossing. If you're using an opto coupler for the trigger, it may have a certain amount of response time. For most of my work, I leave about 500uS after zero crossing before triggering and remove the trigger at least 500uS before zero crossing.