In the gemara, R' Amram, if reuven gives shimon a watch to hold for him and receives a contract in return verifying that shimon has his watch - is shimon believed to say that he returned the watch even though reuven still has the contract (in a case of a loan the holder of the contract is always believed, but in the case of a pikadon the gemara has a question). R' Chisda says that shimon is believed to say that he returned the pikadon. But, the gemara clearly states that the entire discussion whether shimon is believed is only when he claims to have returned it which contradicts the shtar being held by reuven, but if shimon claims that an o'nes occurred to the item, all agree shimon is definitely believed.
Based on this Tosafos asks, why does the gemara assume that even when shimon would die, reuven can use the contract to collect from his children. Although, the claim that shimon would have used, that he returned the item, would not exempt his children from paying, but had shimon claimed that an o'nes occurred he would have been believed. Therefore, we should claim for the yesomim that perhaps on o'nes occurred to the item, to exempt them from paying? Tosafos answers that we never claim "o'nes" for yesomin, because it is not common. The Ri"f in Baba Metzia (perek hamikabel) holds that we do make the claim of o'nes for yesomim. According to the Ri"f, the question of tosafos returns, why don't we exempt the yesomim of shimon from paying reuven based on the claim of o'nes?
The Steipler (34) elaborates on this issue. His basic premise based on a shita mikubetzes on 5b is that one who is holding a contract is considered as if he is holding the actual money. The Shita uses this in the context where a person holding a contract wants to use a migu to take money away from someone else, we don't consider it a mi'gu l'hotzi, because the one holding the contract is not considered to be "motzi" from the other. Similarly, he suggests that one holding the contract may have a stronger claim on the money than the one holding the money, so that the holder of the money cannot use mi'gu to retain the money, because we view him as being "motzi" money from the ba'al ha'shtar. If we consider the orphans of Shimon to be motzi from reuven, since reuven has a contract, it makes sense that we would not claim o'nes for the yesomim to retain the money, because their retaining is in essence being "motzi" from the ba'al ha'shtar. Just as we don't allow mi'gu to be "motzi" money from someone, we also don't claim for yesomim to collect money, only to retain money. Since we view reuven, the ba'al ha'shtar as being muchzak, the yesomim are being motzi, therefore in this case we cannot claim o'nes for them.