The pilot episode is always the most difficult to write because it has to achieve so much.
1) It needs to establish the characters in such a way that you understand them and care about them enough to want to know more. This a delicate balance, you can't give the audience everyhting, there has to be an element of mystery or discovery about these people. It's like a first date, you have to be fascinating but not overpowering.
2) You need to set the story in motion and tease the audience into "having" to know what happens next. the whole pilot leads to that one thing.
The problem with this is
a) It requires huge technical writing skills and experience to craft something that does the whole job perfectly
b) It's almost impossible to achieve the above and make the drama flow well.
What tends to make this even harder is that when writing the pilot the writer doesn't really know the character that well. The character is still being discovered.
The trick therfore is to write a really dog rough pilot episode and then sit down and write the rest of the series. By the end of episode twelve you should know the characters inside and out, plus you've seen where the story is going. At this point you go back and craft your pilot.
I know this sounds like a lot of work, but believe me it is the best way to approach this particular writing task.