You overtrain your arms, and they won't grow.
Here is my split, and technically you still hit arms twice a week due to secondary lifts.
Monday: Bis, tris
Tuesday: legs
wednesday: off
Thursday: Back, traps
Friday: Chest, shoulders.
If you feel that you must do bis or tris on chest or back day do it the opposite. Do chest then bis. And back with tris. There is no reason to hammer out your tris with subpar intensity after cooking them with a chest workout. Just hit your bis and you can give them 100%. Give it a try, it works.