Can't go up/up on serial connection

I'm running into a weird problem I haven't seen before. Granted, I haven't been configuring live equipment very long lol. While setting up a lab I can't go up/up on a serial connection using the default hdlc. I'm up/down. I configured clocking (64000) on the DCE end. It's actually flapping up/down, up/up, up/down, etc.. I first checked all the basic stuff i.e. encapsulation, clocking, ip's, masks. I then checked that keep-alives were enabled, which I've seen cause this before, but they're set on both.

Then, I simply switched the cable so that the DCE side was on the other router, and set the clocking for that side, with no other changes. Now they come up/up and I can ping across..

So I switched the cable back to the way that wouldn't work, and messed with different clocking settings, but none work. I've been messing around with it for the last hour or so lol. I'm going to move on with the DCE-DTE cable arranged the way it is now, because it works, but am really curious why it wouldn't work the other way... So in conclusion:

R1(DCE, 64000)

(DTE)R2 = up/down

then, with no other changes,

R1(DTE)

(DCE, 64000)R2 = up/up

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nerdydad

I've run into the exact same situation before, i pretty much assumed it was my cheap ebay cable and moved on, maybe I should have dug deeper. I have recently rewired the lab and the issue never came back..