Guild Wars 2

The animation of youtube watched two things and referred to it
I said that I was able to easily clear the first.
When I do it once again, possibly I may easily clear it.
But there is not the motivation anymore.
I suggest the possibility that I cannot clear in some kind of factors.

I tried the method many times, but cannot do it.
I tried it for approximately two hours in various timings
However, I did not make a hit.

In the Living World Season 2
1.Gates of Maguuma. – 4. Cornered

I was able to destroy the first shield immediately.
But I cannot break the second shield.

I checked it in various ways and made trial and error for two hours.
I seem to match one of a beginning in the same way, is there this in the bug?