I'm not a user of Passenger Assist but used to be the one arranging it (including for Avanti West Coast services).
As a whole, these issues happen because of communication problems or staffing numbers.
You should be told where to go on arrival and how long before your train leaves. Equally, at the other end, the staff should know where you are and whether to meet you at your seat or on the platform.
I know that this can take time at Euston - especially if multiple people need assistance and there's only one member of staff to help per service.
If you booked through the app, Passenger Assist website or over the phone with Avanti West Coast, it'll be one of their agents, so it'll be easier to investigate what's happened - in other words, was it agent error or an issue on the ground.
Rest assured - the complaints are taken seriously and you'll be due a refund of your tickets on each affected journey (so everything if it failed both ways). The contact centre will chase the stations for an explanation - but sometimes this explanation just doesn't materialise. I've had to do it myself and it is immensely frustrating.
As a whole, it is a great service but the consistency is where it lacks. Every operator seems to have a slightly different policy and way of working. There needs to be a consistent way of marking people as on board a service (it exists - but usage is patchy) and ensuring that staff are properly assigned on the system so that there's accountability.
It's been quite a while since I had any involvement with Passenger Assist - but I hope this explains a little about what goes on behind the scenes.