Why is it so hard? Poor systems, poor processes, poor staff? Might be an availability aspect as well, but the fact the telephone staff are saying they were never ordered makes me think this isn't merely an availability issue.
However, I suspect a cost problem - when marketing created the Volt offer that included free pods, all they thought about was new account volumes, and meeting the company's cross-selling ambitions. They probably never properly factored in the cost of the pods and the take-up. There's a hardware cost for the pod that's probably around the £50 mark, but there's also likely to be a licencing fee as well since the Hub has to run Plume router emulation software. All that eats into VM's meagre margins, and those are next to non-existent for new customer discounted offers. I reason that as a result the pods are costing VM a lot more money than they bargained for, and so the company is seeking to reduce the number it hands out. Having under-invested in the wifi capabilities of their hubs for years, VM saw Plume as a quick, easy solution, now it's a bit of a cuckoo in the nest - they can't get rid of it easily, and they're beholden to a private-equity wifi business that in a few years has gone from nowhere to having a valuation about 20% of the entirety of the Liberty Global group (VM's owner), and still looking to squeeze more value from customers in order to boost valuation for trade sale or a new listing.
Whilst this might sound a touch cynical, it's actually not too different to what VM did with the old (fire hazard) powerline wifi boosters, where after marketing "free boosters" for years, the company decided the cost was too high and unilaterally closed the offer to new requests, and even admitted that this was due to the cost. But at least in that situation they hadn't enslaved themselves to a rapacious private equity business.
However, that's all moot: You've come to the best place, and the helpful UK based forum staff should be able to get this sorted out for you.