I have searched many dictionaries and cannot find "impressing" listed as an adjective. As a present participle, can it be used as an adjective. I know "impressive" is the traditional adjective form, but I have seen "impressing" used as an adjective and I wonder if this is grammatically right. If you can't use "impressing" as an adjective, can you explain to me why not? Thanks in advance for any guidance you might have regarding this issue.
No, there is no such adjective in standard English as 'impressing'.
As for 'why not', the answer is imply the existence of the dedicated adjective form 'impressive', rendering such a form superfluous.