I have tried to contact them by whois' contact information, but never got response.
Did you make an offer? A general "is this domain for sale?" from an unknown person is likely to be ignored by many domain portfolio offers.
Decide the maximum you would pay for it, then make a lower offer (with time limit) to the whois email first, then follow up with phone or post if you're really keen.
But be sure you state the price (US$) you're offering, and assume some haggling will follow if the seller is interested. For many domain sellers, the only way to see if a potential buyer is serious is to get them to name a price.
Having said that, there are a few owners of very large collections of domains that never seem to sell at any price. Strange, especially if they are just parked.