I've notice that Senior members rank higher when answering. Ideally this sounds like a good idea, however, I have had two experiences where a senior member has taken my idea rephrased it slightly and submitted it as his own. My answers received no votes, while a senior member due to their ranking gets credit for my solution.
I have seen that time, and time again. That the best answer (in my opinion) will get more up-votes than other answers, regardless of when they were posted.
Frankly, I sometime prefer to be the last person to the party. At that point, it doesn't matter how long you spend carefully crafting your answer. I have on some occasions taken hours to carefully craft an answer, that later ended up being the most up-voted answer. Quite often these answers will also become the accepted answer.
So my recommendation to you is to stop worrying about other users, and just make sure your answer is as good, as you can possibly make it.
Also if I come across similar answers by two users, one of which has very little rep, I will be more likely to vote on the one from the user with the lower amount of rep. Which is the opposite of what you say you are seeing.
One thing to bear in mind is that just because you've submitted your answer before someone else doesn't mean they've seen your answer before posting.
Quite often I've posted an answer only to see that someone else has already posted something similar. If my answer doesn't contain anything beyond theirs, I'll delete mine... if I think mine explains the same thing better, or adds extra information, I'll leave mine up. The poster of the first answer may think that I've "stolen" their answer - but in reality it's just two minds thinking alike, which should hardly be a surprise to anyone.
I've notice that Senior members rank higher when answering.
Not true. Answers are sorted based on votes and the order of answers with similar vote count is randomized. There are more than enough cases that prove a low-rep member can easily outscore a high-rep member.
I have had two experiences where a senior member has taken my idea...
Note that there may be not a million best ways to accomplish something. Sometimes it's just a couple good ways to do something and you can never be sure that someone has taken your idea merely because it were there first. They may have reached the idea independently. Never ignore this possibility before accusing someone.
...rephrased it slightly and submitted it as his own.
Well, a rephrased answer is no longer identical to your answer, in which case, it's perfectly possible that a third party believes one is better than the other.
Yes, "answer stealing" is a problem and the order randomization has made the problem a little worse. This has been discussed to death in meta before and is by no means limited to people in a certain rep category.
That said, like many things in Stack Overflow and real life, there's not likely to be a 100% practical solution for some problems. Just ignore it and go on to your next question.
I briefly looked through all the answers you've given on SO so far.
Is it possible that other users provided better or more usable answers?
I'm sure you're trying your best to be helpful, which is great. A good answer often comes down to little things like code formatting, readability, and understandability. Try working on improving these things in your answers and I think you will see an improvement in your response rate.
I agree that answers given by high-reputation users tend to be upvoted slightly more quickly and easily than those from others. That's often unfair, but it's human. At the end of the day, as far as I can see, the best answer still counts most on SO in most cases.
The suspicion of "stolen" answers I find unfounded. I can see nothing supporting that allegation. To most technical questions there is a limited number of possible solutions, and it is likely that people will come up with answers similar to yours. It happens to me, and I'm sure most people here, all the time.