When the Poké Radar is used, the game generates four "rings" around the player, with the first ring consisting of the eight squares surrounding the player, the second ring consisting of the 16 squares surrounding the first ring, and so on for the third and fourth rings. One random square in each ring will be selected, and if that square is a patch of grass, that patch will shake. Therefore, the maximum number of patches that can shake with each use of the Poké Radar is four; if fewer than four patches shake, then one or more of the randomly selected squares were not patches of grass. If all four squares did not contain grass, then the game will display the message "The grassy patch remained silent..." and the chain will automatically break.
A 1 x 4 patch should look like this:
If you are standing say here:
*O is the player
Then it is only possible for two patches of grass to shake:
X X O X
X X O X
If none of these patches are selected, then the chain will fail. So obviously, fields of grass that have a larger area have a lower chance of breaking your chain.
What is the exact chance?
If the grass area is big, then you should get at least one shake each time, but with smaller areas, there is a chance there will be no shakes, breaking your chain.
For the first ring, there is a 1/8 chance of a certain patch being selected, if there are eight patches of grass surrounding the player.
If the player is standing here XXXO then there is a 12.5% chance of that the patch of grass will shake, meaning there is a 87.5% chance of the chain breaking.
If it breaks, there is still a 6.25% chance of the second ring hitting home. The third ring has a 3.125% chance of hitting the farthest patch.
Add that up, and altogether you have a 21.875% chance of your chain not breaking, so a 78.125% chance of the chain breaking.
As for Magnet Pull, I was under the impression that while in a chain you can only encounter the specific Pokémon being chained, until it breaks. Or perhaps that is only in X and Y? I didn't read anything about it on Bulbapedia though, so I believe Magnet Pull doesn't affect chaining.
If the player was standing here XXOX then the chance is 68.75
You can't chain on a 1x1 patch of grass.
There is an 87.5% chance of a chain breaking on a 1x2 patch.
If the player is standing here XOX there is a 75% chance your chain will fail on a 1x3 patch.
If the player is standing here XXO then there is an 81.25% chance of a chain breaking.
So to sum it up, it isn't really strange that your chain broke.Unless I have it all wrong.
Please tell me if I have got it all wrong or my maths is wrong XD