Paraphrase of an email I sent to a colleague

I noticed a problem with System A. I have determined that it is not caused by X. I suspect that it is being caused by Y. (since you are in charge of Y) Can you please take a look at it?

(The part in parentheses was not included in my email because this colleague knows full well he is in charge of Y... it should be understood that's why I was sending him the email)

I run into the colleague half a day later, and say, "did you look at the problem with System A yet?" and he answers, "Oh, yeah, I got your email about System A being broken but I assumed it was probably caused by X, so I was waiting for you to check on that."

Obviously my message did not get communicated. This is an all-too-common result of emails I send and is very discouraging... yet, I don't know how I could make my emails any clearer. They are always as brief, clear, and to the point as I can make them. Any suggestions? Maybe there is something inherent about the medium of email that is just not effective for these types of communications?