Make sure you're pricing is up to date. Remember, ticket prices don't change unless you change them. If you set those routes up years ago, all your costs have gone up since then as well as inflation. The suggested price is now a lot higher than it was then.
Coming from someone who flies them more than just about anyone in the game, I will say that the A310 is NOT a plane for the less-experienced player. It has a very low pax load for its operating costs. They're often bought by less-experienced players because of the short production queue -- but that queue is short for a reason. I buy them frequently for the same reasons, but I know when to get rid of them as well; so I buy them only to cover me until I can take delivery of superior options.
If you're flying it on shorter routes then the A321 is a FAR superior option. If you're flying it on long, thin routes that the A310 is designed for, you're just going to flat-out have a hard time of it, particularly if you're leasing as it's quite expensive. You've got a plane that carries fully 1/3rd fewer passengers than an A330 but uses just as much fuel, costs just as much to crew and operate, and costs 80% as much. There really isn't better options out there though -- in real-life, and therefore in the game, there's not much of a market for aircraft with a 5000NM range that only carry 175pax because the costs per passenger to fly that far with so few people is just too much.