The offers you made might or might not be--or be considered to be--lowball offers.
Let's unravel the issues.
First, you should be making offers based on the values of the homes. Just because a house is listed for $515,000...or $550,000...or whatever, doesn't mean that's what it's worth. It could be worth more or less. If, for instance, that $515,000 really is worth $525,000, then an offer of $520,000 (even though it's above the asking price) would be a reasonable value to you. So, without knowing the comps, it's impossible to say whether your offers were out of line or not.
Second, there's the perception of a "low ball" offer. Sellers have different perceptions. Some already think they're "giving their house away" at whatever the list price is. I've talked to many owners who explain, in excruciating detail, why their property is worth so much more than identical properties on the same street. In one case, it was because the owner spent two months painting one bedroom and the dining room. In another case, it was because their backyard was a tiny bit larger. These folks will consider any offer below a full price offer to be unacceptable. In other cases, though, sellers are willing to take substantially less than the list price, and they're happy to do so.
All you really can do is determine the home's true value. Then, depending on how much you want the house (and perhaps your perception of the degree of motivation of the seller), keep your offer at that level, or drop it down some. Your Realtor can help you out with that though, ultimately, it's your decision.
In addition, for any properties still on the market (as you say those are), there's nothing to stop you from making another offer. Banks, for instance, will reduce the prices on REOs until they sell. And individual owners may become more motivated if their house doesn't sell in, say, 60 or 90 days.