I have had very little success with the 1.4 TC on ƒ6.3 lenses.
As a result, my last two lens purchases have been ƒ2.8 lenses.
I keep telling folks, if you buy a 150-600 or 50-500 or any ƒ6.3 lens you can't reliably use a TC but do people listen?
I know this is tough for the low end users, but, if you are going to use a TC, your best options are ƒ2.8 glass, with ƒ4 glass being the second choice. Your AF system needs light to work, and ƒ6.3 with a TC wide open just isn't enough light. And there's nothing you can do with about it.
I whined and cried about it for years before i accepted that, it's not fair, it's not negotiable, it just is what it is.