Here are several examples:
ANH
Vader is able to detect his old master who he has not seen in a generation yet cannot detect his own children (Leia and Later Luke) throughout this movie. Yet in ROTJ Vader can detect Luke who is presumably much better skilled at hiding his presence than the untrained Luke in ANH over a large distance, even better than the Emperor.
Does Force detection require some concrete knowldege of a connection? In ESB Vader can detect Luke fairly easily (in one instance over lightyears while watching a videoclip of Hoth) only after learning that Luke is his son. Sort of a weird limitation especially in light of Yoda and Obi Wan's concerns in ROTS that the Sith may be able to detect the children.
ANH
This point requires a presumption. That the mindprobe mentioned by Vader during his interrogation of Leia is a Force abilty. If the mindprobe is indeed a Force ability then why was he not able to detect Leia's connection to him (she remembered her mother according to ROTJ but that can always be chalked up to Bail's wife) and more importantly Vader is able to pluck things from Luke's mind in ROTJ ("Sister. SO you have a twin sister") without even being able to see Luke yet cannot pluck out the location of the plans from an untrained Leia in the same room under duress?
ESB
Always in motion is the future. Yoda cannot detect the fate of Luke's friends yet Palpatine can see the entire length and breadth of a Galactic War throughout the prequels and manipulates events from ESB onward to his advantage.
Some powerful ally you got there Yoda.

ESB
C'mon. Vader can detect Luke over planetary distances and suddenly when he needs that abilty most he 'thinks' Luke's on the Millenium Falcon? His 'mistake' allows Luke time to be trained.
ROTJ/Prequel Era
Palpatine is able to coordinate the destruction of the Jedi Order and the Fall of the Republic with near perfect precision through the use of his far seeing ability. He is able to out see such masters as Yoda, Mace Windu and the collective Jedi High Council.
YET
When the chips are down his far seeing fails him precisely when he needs it most - in regards to his hold over Anakin and Luke. He sees just far enough to get Luke on the DSII and place everything in position for his defeat at the hands of Anakin. As if the Force had manipulated the Sith and the Jedi - granting a single Sith lord the ability to see better than the Jedi and even cloud their vision yet pulls back just in time to see Palpatine fall.
ROTS
Anakin's precog, presumbaly better than Obi Wan's cannot tell him jumping at Obi Wan is a BAD idea at this point in the duel?
Prequel Era
Veil of the Darkside. One Sith Lord can dampen 10,000 Jedi including masters like Yoda and Mace Windu. The Jedi outnumbering the Sith cannot seem to do the same in reverse. Yoda in fact resorts to probing the darkside (AOTC) for answers.
These cherry picked examples are meant to provide evidence for the point that the Force somehow manipulates the Jedi and Sith, revealing knowledge that it needs them to use and withholding help precisely when the Force user needs it most.
Or are the Jedi and Sith only seeing what they want to see?
If you think I'm full of shit please discuss.