I know that Jedi can sense one another but why can't Jedi in Star Wars The Clone Wars sense each other. I mean in the books, movies and games they can sense each other. Take season 4 episode 15. One Jedi couldn't tell the that the guy he was arresting was his master that was killed. You should be able to feel when they die or if they are still alive. Can someone give me an answer as to why this is.

It's true that Jedi can sense one another, but the conditions vary. If the area that one of the two Jedi are in is saturated with the Dark Side, then this ability isn't as possible. In the case you're talking of with Anakin and Obi-wan though, the issue is willingness. Obi-wan didn't want Anakin to know who he really was, so he masked his abilities with the Force, thus making it impossible for Anakin to sense him.

It's a plot device.

 

Jedis can sense each other/sense the Dark Side when its convenient for the plot.

 

 

To be fair, we know the Dark Side can conceal itself very well but other than that, there don't seem to be very many solid rules.

That's a good point, but I presume its a mix of Obi-wan is hiding his Force 'signature' and Anakin being too consumed with grief/anger/etc. to properly sense Obi-wan. Or maybe he thinks he's sensing what Obi-wan would want, as he did say "You're lucky that the man you killed would rather you rot in jail". As for Ahsoka, we can assume similar issues, though less grief more 'If you want to gut him, Master, I'll look the other way'.

Assumingly a Jedi can sense another Force User (or any other creature in the universe) providing these conditions:

 

a) Their connection with the Force, how strong they are , like Yoda and Sidius.

 

b) How strong the other is, emmiting aknownigly their own presence through the Force ether.

 

c) Special connection between two people that provide a "bonus" even if their Force connection is low for such a feat.