I have 2 Jackmanii's in front of my front porch which are well established, 9 year old plants. A couple of years ago, one of them suddenly started having several stems (but not all) having a problem that looked like a nutrient deficiency. Green veins and spots with yellowing in between. I fertilized it with a liquid fertilizer mixed with epsom salt. It made no difference. The leaves got progressively more discolored over a period of days, then turned brown and died. This happened to that plant 2 years in a row. This year it hasn't had the problem yet.
However, this year, the other Jackmanii is doing the same thing. From reading the descriptions of clematis wilt, it doesn't sound like that. The stems don't wilt first. The leaves discolor, then eventually die. I'm posting pictures. The third picture is one where it's pretty advanced and about to turn completely brown. You can see some dead leaves around it, and also some perfectly healthy leaves on other stems. I hope someone can help! I don't know what to do about this problem that keeps re-occuring.