Some of my recent transactions are not confirmed and if I try to use a tool such as this or this, I get an error saying "Inconsistent subtangle" or "Cannot find tail" or "Inconsistent tips pair selected". What do these mean and how can I avoid them?
"Inconsistent subtangle" - transaction you want to promote is referencing an invalid transaction (or one that was confirmed in other reattachment). That means you have to use reattach.
"Cannot find tail" - not sure, maybe this node doesn't see this transaction or it isn't a tail (transaction with index 0 in the bundle)
"Inconsistent tips pair selected" - it's an error from the node when it can't find consistent tips. You can just try again.