Whitney  in his grammar section 227 says about the doubling of "ch".

"As a general rule ch is not to be allowed by the grammarians to stand in that form after vowels but is to be doubled becoming cch (which in the manuscripts is sometimes written chch). . .According to Panini ch is to be doubled within a word after a long or a short vowel."

But if you look in his "Roots, Verb-forms and Derivatives" at the entry  for iṣ, ich  nowhere does he double "ch" not even after a short vowel rather he has ichati, ichaka,  ichā and ichu  . Does anyone know why for this root in all his examples he didn't double ch after vowels?

Thanks,