when you have a lot of Whatsapp convos going then your attention tends to stick with the more interesting and engaging convos. As soon as you're off the first page of Whatsapp convos then you're history.
I think there is a lot in this. Some girls and guys just lose track. If you're going to be successful as a SB or SD, then you really need to keep up with several convos at once on WA, ie respond in a timely manner if you're vaguely interested in them, remember their names and what they've told you/haven't told you (or at least scroll back to see), not getting confused with other people.
If you can only handle one or two convos at once, then you run the risk of being left empty handed when those leads go nowhere.
Lots of girls defend their slow replies by saying "sorry I've been busy" which is lame. If someone is that busy to even exchange the odd text then they aren't going to find time for a meet, and shouldn't be conversing in the first place or be on SA at all.
But, of course, it's bollocks. It takes literally 1 or 2 seconds to write a message and even the busiest people find time to meet - and don't say they're busy. The vast majority of these girls really aren't that busy; they just can't be bothered. As has been said, you can see that most of them often online, busy chatting to other people, including guys off SA, rather than you.