One of three things could be the cause:
Part of a dollar bill/piece of paper/foreign object is holding the "anti-pullback" lever stuck.
Dirty optics (phototransistor and infrared LED) on the Lower Cell & Harness circuit board. This is the most common problem. Take the board off and clean the surface of the phototransistor and LED with a cotton swab moistened with your favorite household spray cleaner like 409, Fantastic, Windex, etc.
Defective Lower Cell & Harness Assembly (costs around $80). Must be ordered from the parts department of your local Rowe distributor or from one of the national coin-op parts houses like Happ Controls or Betson.
There are two different service manuals issued depending on the application you use the bill acceptor for:
For Jukebox and amusement game applications, use Field Service Manual & Parts Catalog # 21922101.
For Vending Machine and Universal Applications, use Field Service Manual & Parts Catalog # 25520901
There are two update kits available which allow this bill acceptor to take the latest $5 bills. The kit to order depends on the date the bill acceptor was manufactured. If the CBA-2 is older than 1991 (date code first two digits are year) then you need kit # 27039804 which includes a new main circuit board assembly, water shield. and additional items to make it current. If it's newer than 1992 then you order kit # 65100301 which includes a new main PCB assembly and water shield.
Early versions of the CBA-2 did not have any dip switches or motor speed adjustment trimpots on the main board. It is suggested for improved operation to install one of the above update kits.