I am a regular user of stack-overflow, i have seen members up voting small answers which takes minimum amount of time to identify the problem and solved, whereas the answers which took lot of effort and time receives only one or two up votes, sometimes there won't be up votes too.Why users are not willing to up vote answers which deserves upvotes?
Basically it's because it's a simple answer.
It's an example of Parkinson's Law of Triviality which states that organisations give disproportionate weight to trivial issues.
What this means in this case is that that because everybody can understand both the simple question and answer they are more likely to up-vote the correct answer(s). Questions and/or answers that require more thought to understand won't attract nearly so many votes.
It's one of the reasons that people answer these questions rather than voting to close as duplicates (which they often are).