We say on here over and over wait 72 hours before doing anything, it's even a sticky, that fermentation can take up to 72 hours to even begin.
That's why in all those threads on here we recommend after 72 to hours, to take a hydrometer reading before doing anything.
And by visible signs we don't mean bubbling of the airlock, we mean krauzen or more importantly hydrometer readings.
Fermentation is not always "dynamic," just because you don't SEE anything happenning, doesn't mean that anything's wrong, and also doesn't mean that the yeast are still not working dilligantly away, doing what they've been doing for over 4,000 years....
The only way to truly know what is going on in your fermenter is with your hydrometer.
Like I said here in my blog, which I encourage you to read, Think evaluation before action
you sure as HELL wouldn't want a doctor to start cutting on you unless he used the proper diagnostic instuments like x-rays first, right? You wouldn't want him to just take a look in your eyes briefly and say "I'm cutting into your chest first thing in the morning." You would want them to use the right diagnostic tools
before the slice and dice, right? You'd cry malpractice, I would hope, if they didn't say they were sending you for an MRI and other things before going in.....
Thinking about pitching more yeast before taking a hydrometer reading after 72 hours, is the same thing....thinking about a "cure" before we even know if the beer is even "sick."
And especially if you didn't make a starter. With any liquid yeast you should make a starter. Especially one where the smack pack didn't inflate fully, that way you would maximize the cell count of viable cells in the packet. If with time we can grow a huge starter from the dregs at the bottom of a bottle of beer...you can insure a good amount of viable yeast by making a starter...and since you didn't you should expect a longer lag time.
One of the reason there is a lag time anyway, is because the yeast end up having to reproduce to eat all the sugar in 5 gallons.