Pay extremely close attention to the language used on
Disengage.
"Consumes the Parried Debuff on Hitting Enemies to release a shockwave and grant you a Frenzy Charge"
Note that this debuff is on the monster. It is not consuming a buff that was placed upon you as a reward for parrying, it is consuming a debuff that was placed upon the monster as a punishment for it having a move parried.
This means your
Disengage needs to hit the EXACT monster you parried a strike or projectile from, rather than firing into the nearby pack of monsters.
This is the first reason you frequently successfully execute Parries but fail to gain a Frenzy Charge.
The second reason is that
Parry can only parry projectiles fired at point blank range (0-1 meter). If you are struck by a projectile fired from 2 meters away and it either deals negligible damage or your Evasion Rating causes it to miss, you may believe you executed a Parry when you did not, and so you may initiate
Disengage with no available targets that would grant the shockwave and the Frenzy Charge.
A third reason is that rarely,
Parry's retalliatory damage will outright slay the monster you parried. This is likely to happen more often since 0.2.0 Hotfix 6 applied sweeping nerfs to monster HP in the campaign.