Guinea pigs need veges every day,1 cup full per pig to be exact and variety needs to be a minimum of 3 veges a day,mix it up as much as you can have a look at the food stickys to help with range.
They also need unlimited hay meadow hay will be fine or you can go to companys like "the hay experts" and "oxbow",there teeth will get incredibly long and prevent them from eating without hay.
Wrong diet may be a contributing factor to behaviour,some petshops pigs are also highly exposed and therefore get scared and dont want to be held,thats why its always better to adopt from a rescue,
Guinea pigs also need friends so that is probably part of the reason to,a rescue can help pair a guinea of the same sex correctly so they have minimal chance of falling out.
Guinea pigs live in heards in the wild so a lone guinea pig is usually a sad one.
as for handling it can take months for some pigs and days for others they are all different,the best think to do is be calm and speak in a low voice,take your time and eventually he/she should reward you with love,the above requirements with diet ect/friends make for much happier piggies,
Also for 2 piggies they would need a cage thats 2 x 4 in C&C cage sizes, petstore cages are rarely big enough xx