Re: something is wrong with the infiltrator and the vanguard in multiplayer...

Enemies can still hear you while cloaked and shoot at the last position they saw / heard you.

The stagger ability is depending on the force of your charge. So, if you didn't skill the power-bonus in the passive ability tree and / or left out force-bonuses of charge it can happen that enemies stagger less often, especially on higher difficulties like Gold or Platinum, where the stagger-threshold is higher.