For many babies and sleep, it’s typically a timing issue; you have to hit that exact moment.
I do feel the beginning of the yawn, may indicate that you missed the sleep window. At that point, they’re overtired and too wired to fall asleep.
I am not a fan of watching for baby’s sleepy signs as many babies don’t show any sign at all until it’s too late! I prefer the method of timing based on the babies age. For example, many babies around 6 months can be comfortably awake for 2-2.5 hours. I find timing naps and sleep so they are not awake longer than that time is often accurate. At 4 months, babies typically fall into a nap schedule consisting of two longer naps, and a cat nap a day (one in the morning and another two in the afternoon).
Scheduling naps and sleep is imperative for good sleep habits. Poor naps, almost always make for very poor night time sleep.
Let her fuss
Think about it: Do you fall asleep the minute your head hits the pillow? Probably not. Well, neither does your baby. So when you go to pick her up at the first whimper, you may be distracting her from falling asleep, or even waking her up.
Sleep baby sleep
Try to hold back from checking on her for a few minutes. If you don’t give your baby a chance to calm herself, she won’t learn to do it. Check on her with the baby monitor.