While turning the impeller from inside tub have someone else watch the fan blades on the motor with a mirror under the machine.

Most likely you will find when turning the impeller the motor isn't actually turning, thus when motor is running, impeller isn't turning or is only turning very slowly from friction with back portion of impeller which front vane section has broken away from.

Adding a little to this, make sure when turning the impeller from inside the tub area not to push inward with too much pressure while turning because this will shove the two impeller halves back together and possible make the actual motor turn and make you think the impeller is OK when in-fact it is the problem.

Here's the part link for the circulation impeller kit: http://www.repairclinic.com/PartDetail/Impeller-and-Seal-Kit/167085/935404

Here's how to install it:

Source: Bosch Dishwasher SHU5305UC/U12 low/no circulation pump pressure