I run a small Bitcoin related service in which bitcoins get sent out my users regularly throughout each day via my bitcoind node. Once or twice a week some of the transactions don't seem to be properly broadcast, and when pushing manually using Blockchain's pushtx tool, Blockchain will respond with Unable to find inputs: [...] (where the ... is one or more txids)

Pushing the inputs listed will usually solve the issue, but why is this necessary in the first place? How can I fix it? I hope I'm describing the issue well enough; I'm finding it hard to explain.