I have a Teddy pig who I would class as a 'tweaker'. She's bold and brave and knows what she wants. She's very friendly, but if I annoy her (clipping her nails, holding her too long before putting her on my lap, just not in the mood for a fuss etc) she will place her teeth on me and start to apply pressure. It's not a true bite, just her way of telling me she isn't liking what I am doing with her at that time. She loves a fuss and a rub behind her ears .... except when she doesn't, and again she'll turn and threat-bite me if she's not in the mood (again it's a message, not a real bite).
It's very common indeed for piggies to resist being picked up. The advice of picking them up in a cosy or a tub is good advice, and it can be calmer for both you and the piggy when they really hate the 'grabbing hands'. Some of these same piggies are perfectly fine once on the lap, and some never actually enjoy lap time. It is simply a myth that all piggies love lap time. It's nice when they do, but certainly there are plenty who do not.
However you can bond well with you pigs even if you don't get to cuddle them as much as you would have hoped. I had one piggy who took biggest part of a year to warm up to me, then all of a sudden she really liked me. We'd nose-sniff each other through the bars, she'd sit on my slippers and chew the little ribbon, she'd jump onto my lap when I sat in the play pen with them, wanting some hand-fed tid-bits.... BUT she would not allow me to touch her. We had a bond, a lovely bond, but it was a strictly no-touch bond (made nail-clipping awkward, but never mind!).
As mentioned, hand feeding and picking them up in a less stressful way are ways to help them to relax with you, and often an enjoyment of lap-time can grow, but not always. It can take quite a while with some of them, months, sometimes many months, but the realisation that a piggy who does not want to be touched doesn't always mean an inability to bond with you is really helpful. Some pigs really do relish your attention and interaction, but don't ever want cuddles.
i don't know what i'm doing wrong, am i just really unlucky and got two mean guinea pigs? my one piggie squirms a bunch whenever i try to hold her and only sometimes bites, but my other one will keep biting you until you put her down. i try to hold them everyday but it hasn't helped at all. i've also tried feeding them while they are on my lap but they won't eat. please help! 
x