LG G6 H870 TWRP not sticking



I followed the unlock bootloader from android 101 series and it worked great.

I tried to flash TWRP and I got the teamwin screen on reboot but nothing happened after yes/yes. Next time I tried it wiped phone so I assume TWRP hadn’t stuck.

The files available on TWRP are:
PGP Signature twrp-3.2.1-0-h870.img.asc

I only downloaded and flashed the first of these two as the universal instructions state that the second file should only be flashed if it is a zip file.

Any help appreciated as I’ve tried it three times and I must be doing something wrong…but I followed the instructions to the letter.

MacBook Pro. Also I tweeted this to you before I realised you had a helpful forum. Apologies.