sk10009, I dont understand why its a problem, TOFD compression probes are readily available as standard for all commercial TOFD scanning systems.

Anyway:

Diffraction is stronger for longitudinal waves than for shear waves.

For identical frequencies and crystal diameters, beam spread is greater for compression wave due to its longer wavelength. The TOFD method requires "flooding" the material under test with sound with a wide beam. Thus also smaller crystal size is desirable.

Due to higher velocity, compression waves will always reach reflectors before shear waves. The reflected/diffracted shear waves in the material are therefore "lost" in the mode conversion signals coming from the backwall of the test specimen leaving a clear signal display for inspection zone.