Suppressed premises.
Traditionally, logicians have called premises that are not stated but are needed (to make the argument valid and explain how it works) suppressed premises. An argument depending on suppressed premises is called an enthymeme and is said to be enthymematic.
After a reasonable search for those suppressed premises that would show the argument to be sound, we should not blame ourselves if we fail to find them. Rather, the blame shifts to the person who formulated the argument for not doing so clearly.