but to answer your specific question; "Is it normal for a virus to slip through and then be detected later?", I think the answer is "Yes".
But, providing the antivirus program is up to date, with a definition file that knows about the virus in question, it will detect it on either of 2 events, namely, when a scan is performed to look for virus files, (which is what you've done), or when the virus file itself tried to execute, i.e. perform it's nasty deed.
If the antivirus files on the program are up to date and knows about the virus, then, I think, it will detect it as the virus is saved to disc.
As usual, I'm open to correction. But that's my understanding of how the nasties are "policed".
And I stand corrected but is it not good advice to switch of "system restore" before running your AV as these little tinkers can hide their selfs in your restore points oh mabye im wrong but thought it was worth suggesting. Ps. but dont forget to turn it back on after.