15-20 years ago, my thinking was similar to your own, that is, i "felt" like the wasn't doing enough. Little did i know that even though i was performing just one direct biceps exercise, and one direct triceps movement, i was actually overtraining. It wasn't until i eliminated ALL direct arm work did my arms actually begin to signs of improvement.
The arms receive more than enough stimulation from compound movements such as underhand pulldowns/chins, heavy rows, high pulls, dips, bench presses, shoulder presses etc, to then add additional direct arm work in many cases is just too much. The only time i actually use direct arm work now, is when i'm using pre-exhaustion on the larger muscle groups (as less weight can be employed on the compound movement).
In general Eden, i always found that the less exercise i do (within reason), with greater intensity of effort, the better the results, and even though i "feel" i could do more, i don't, as the purpose of going to the gym is to stimulate growth, not to see how much we can endure. Stimulate, rest and grow!
The most productive program I've found for my arms was this Bill Sahli (a world class hit trainer with 45 years experience) powerbuilding routine, in fact i myself return to using a very similar routine every so often.
Workout A PUSH
1. Weighted Dips
2. Close-grip Bench
3. Squat or Leg Press
Workout B PULL
2. Rows or High Pulls
3. Underhand Pulldowns or Chins
* 4-7 days between workouts (to allow for full systemic recovery)
* After warm-up sets, perform just one set to failure per movement
* Salt in some "high stress" techniques, such as static holds, partial reps, rest pause etc (no more than one of these techniques per workout)