Such munitions often malfunction, simply do to the blasts needed to get them to the target area or the blast to spread them apart. Such Munitions MUST be able to survive such blasts. For example if we are talking of artillery, the munition must survive the blast out of the Cannon, then the blast of the Shell over the target area, then the fall to the ground. That three points of possible malfunction. If the munition is aircraft delivered, it avoids the initial blast out of the Cannon tube (But replaced by some sort of jerk do to the drop from the plane) but the other two causes of failure still exits.
Remember the three evil enemies of electronics, Oil, Water and Vibration. These munitions laying on the ground are exposed to water by just laying on the ground. Vibration from the blasts and the fall to the ground. Oil from plants the munitions land on can also enter and make the munition malfunction.
Now, when I was in the Field Artillery in the 1980s we joked about the shells we were firing. Each ran about $20 a shell. Shells are cheap. On the other hand the fuses ran from $95 each to $300 each (This is the 1980s). The reason was each fuse had to be able to withstand the blast out of the tube and still go off at the target. When we were setting Time Fuses, we were told we could NOT turn the fuse more then three times around, if we did the fuse had to be gently put down and bomb disposal had to be called in to get rid of it. The reason for this is the fuse was designed to go active once it turned three times around AFTER it exited the cannon tube when the Round was fired. It was a delegate design, Timex took pride of its product (Yes, Timex main product was NOT only cheap mechanical watches of the 1930s through the 1970s, but fuses, both used similar mechanical mechanisms).
Even today, the Army has NOT switched from Mechanical time do to the fact it can withstand the blast out of a Cannon and explode over the target (The US appear to have used mechanical time since WWII, through I have read British WWII reports of the British using black powder on its "Timed" fuses during WWII). Given the price of Mechanical time fuses (Timex stopped making watches when the digital watch came in, could no longer compete as to price) I do NOT see Mechanical time pieces being used in such cluster bombs. Thus the devices in the cluster has to be some sort of electronic time piece which are suppose to turn off the cluster bomb after a set time. I do NOT trust such devices given they are exposed to two of electronics enemies (Water and Vibration). It also appears the Army does NOT trust such electronics, even today the preferred way to handle Cluster bombs, even of US manufacture, is to shoot them with a 50 caliber Rifle from at least 100 yards away (IF none available then 7.62 mm rifles from the same distance, but the 7.62 does NOT guarantee that the cluster bomb will go "boom" when hit, the 50 caliber almost always get the cluster bomb to blow).
Sorry, the only report I have found as to the reliability of such cluster bombs turning themselves off indicate a failure rate up to 30%. That was from the Congressional research service dated January 11, 2011:
There appear to be significant discrepancies among failure rate estimates. Some manufacturers claim a submunition failure rate of 2% to 5%, whereas mine clearance specialists have frequently reported failure rates of 10% to 30%. A number of factors influence submunition reliability. These include delivery technique, age of the submunition, air temperature, landing in soft or muddy ground, getting caught in trees and vegetation, and submunitions being damaged after dispersal, or landing in such a manner that their impact fuzes fail to initiate.http://www.fas.org/sgp/crs/weapons/RS22907.pdfSounds like the Military that uses Cluster weapons, exclude from the failure rate anything cluster bomb that did not work (i.e. did NOT turn if self off) if some other reason other then failure of the Mechanism was the cause (i.e. if the Bomb fell into mud, its "Soft" Landing was the cause of the failure, not the failure of the mechanism, if the bomb was damaged during the blast to dispersed the bombs, also not counted for the failure was do to the blast not the mechanism).
On the other hand Mine Clearance Specialists list every bomb that is active for whatever reason as a failure. Classic case of Statistics don't lie, but liars know how to make Statistics. a 30% failure rate is still unacceptable, given the nature of such weapons and they long life (i.e. can be active for decades).