Whi;lst the weather is probably the main problem, I don't think we get really honest help from the suppliers. About 3 years ago I did a series of side-by-side trials of 2 sugar snap varieties and 2 mangetout varieties (all T&M). In every case I got good germination of the latter and very poor with the former. I sent full evidence to T&M and only got a nose-in-the-air kind of reply.
I got three packets of seeds from one supplier: of the ten chilli seeds sown, one germinated; all the Big Boy tomato seeds I sowed germinated, but not a one is a Big Boy plant (I've grown these for several years and know what they should look and taste like); of the melon seeds, all germinated and all have done well. I've pondered on what to do. After all, the chilli seeds not germinating could have been my fault and the melons have done well. But the Big Boy? I'm quite disappointed as I have eight of these plants and the tomatoes are poor: thick-skinned and sour-flavoured and look more like a Moneymaker.
My first packet of parsnip seeds did nothing, but the second packet (bought locally from a supplier who probably sells one packet of parsnip seeds a year apart from this year, with my purchase making it two) came through with a flourish. Both were sown quite late by UK standards, so I don't think the weather was an issue.
Of my carrot seeds, very poor germination on the first lot sown but pretty good with all the others, including Lidl Nantes carrot seed tape bought three years ago for me by Mr Snoop.
So, with some successes within the same family of plants, who's to blame for the failures? Me or the seed supplier. In the case of the Big Boys, I reckon it's the supplier. But it's a winning game for them with all the others: they are bound to think it's my fault, the weather, the slugs, the snails, the spider mite, whatever.