Guild Wars 2

when the guardian got swiftness more than 40sec,then the guardian can not stack the duration of swiftness with any of guardian skill suck as staff 3 and retreat sometimes

That’s intended, it isn’t the only skill to do that. Keeps you from standing there for a few mins stacking minutes of swiftness. Imagine if a party of guardians could stack indefinitely… you’d have hours of swiftness within a few mins. Ideally you would stack staff 3 first, then again while running to add a little, stop when you need to up it again, and use other skills for more swiftness. Helps to have someone else stacking swiftness from a different source. At least it isn’t like Mesmer focus 4 >.<

As Lunacy Solacio said, it’s not a bug, that’s an intended mechanics for boon-stacking. The duration-stacking boons are limited to a maximum of 9 stacks, with the exception of retaliation (5 stacks) and stability (3 stacks).

So, assuming you’re stacking swiftness with multiple guardian’s symbol of swiftness, which applies 4 to 6 seconds of swiftness per pulse (depending on player’s boon duration bonus), you reach the stacking cap at approximately 40-45 sec.
Every other source of swiftness (“Retreat” or warrior’s warhorn “Charge” for example) is then wasted until the first 9 stacks start to wear off.

This is mostly an issue in WvW where swiftness stacking is regularly done, and you should always, if needed, use your “Retreat” before getting stacks of short-duration swiftness from symbols.