I can be very specific on this.
Yates was fired on January 30, 2017. Before she was fired, she told the WH counsel's office that Flynn had substantive conversations with the Russians, about sanctions, and that there was concern he could be blackmailed by them, either with that or something else.
So we know the WH counsel's office knew of this on or before January 30, 2017.
When asked about the substance of that on February 10, 2017, Trump told reporters on Air Force One that he had absolutely no knowledge of it and would look into it.
The issue is therefore three fold.
One, was Trump made aware of Yates' report to WH counsel or or about January 30, 2017. If so, why did he not immediately fire Flynn for having undertaken such a dangerous and remarkably dumb approach?
Two, if Trump was not made aware of this, then what the F is wrong with people in counsel's office? It is obviously critically important for the POTUS to know that his NSA director has had such contact.
Three, if Trump did know prior to February 10, and lied when he told the press on AF1 that he did not know anything about it, and Flynn was forced out only when the heat got to be too much, then Trump has enormous problems moving forward.
Numbers 1 and 3 are grounds for impeachment, no doubt about it.
Number 2 is troubling and another indicia of general incompetence in this WH, but it gets Trump off the hook, personally.