Pro Mini driving me mad

Hello

I have an UNO set up as a programmer for my Pro Minis.... the way I always do it

RX-RX, TX-TX, Ground - Ground.
Uno reset held low.

Always works fine.

Yesterday, it all failed and I just cannot fix it.

Changed out the UNO twice and tried 3 new Pro Minis.

Nothing connected to the Pro mini except RX,TX and a supply. Removed all the other circuitry.

The code will read (progress bar runs across the screen), but then hangs at the write part of the process.
It shows 'write', and then the progress bar just hangs.

Any ideas? I spend more time troubleshooting programming these damn things than I do the actual code

Have you disconnected the Pro Mini, released the Uno from Reset and checked that you can still upload sketches to the Uno like normal?

Yes.
Turns out to be part of the code???

Complied fine, yet once I commented out the Servo pos command, it uploaded. Bizarre.
If you uncomment the line, it fails again.

The code is a DMX receiver, and all the servo libraries I have tried so far (MegaServo, Servo etc) all interfere with the DMX code.
Looks like I need a Mini Pro just decoding the DMX, and then talking to a servo driver board.

Just trying it on a Raspberry Pi Zero instead

What you describe should not work for reprogramming a pro mini - there's no way for it to trip the reset on the pro mini and send it into the bootloader. You'd have to either press reset on the pro mini at the exact right time in the upload process, or remove the chip from the uno and connect reset to reset.

Why don't you spend $1.89 and buy a real serial adapter that you can just connect to it? https://www.ebay.com/itm/191146797242 (no specific endorsement of this vendor, but that model of adapter is a good one - the black ones with the voltage switch are excellent. Also the green ones with micro USB connector and voltage switch are fine. Uses CH340G driver (google will find it) if you don't have it installed). Note that with a serial adapter, TX goes to RX and vise versa - the reason that you do TX-TX and RX-RX with uno is that the pins are marked with respect to the '328p on the Uno - the pin marked RX is connected to TX of the on-board serial adapter.