It's common at low levels for a variety of reasons.
On your end, it could be a lack of gear. You especially want to keep your weapon up to date, but just in general your ability to generate threat for a long time is going to notably rely on your gear. Also, don't just flash - use your 1-2 combo that generates aggro as well on anything the DPS are focusing! Flash might be good at generating aggro but a determined DPS can still outpace that if that's all you're using.

On their end, the most likely candidate for stealing aggro is just refusing to target the same thing you are. At high levels there's other issues that can arise from burst damage, but that's not likely at low levels.