Ultrasonic cleaner control with Arduino
Figure 1. Poor bridge driver circuit.

The problem is that of a badly designed bridge driver. The CD4013 is driven from a 5 V supply so the outputs (1) and (2) can only switch between 0 and +5 V. T1 and T2 are configured as voltage followers so the emitter can only reach 5 V less 0.7 V, the voltage drop across the base-emitter junction. The highest voltage at (3) or (4) is 4.3 V despite being powered from 25 V.

Note that the original circuit is all 12 V so the bridge may be able to switch between about 1 and 11 V.

You'll need to use a proper H-bridge driver circuit.