Slapping - Some instruments easier than other?

I'm trying to get a crowd-sourced opinion of WHAT goes on when slapping on different basses.

My Spectors are a Slapper's Delight! Clear tone on every string, up and down the neck.

My Ibanez active Jazz (TR500) is a mixed bag. The 3rd and 4th strings give the expected, solid, clacky thump. 1st and 2nd string, is much more difficult to get a clear tone. I have to curve my wrist and thumb to get a well-defined note.

Both instruments have a similar neck-profile.
The main physical differences:
Jazz is bolt-on. Spector is neck-thru.
Jazz is 22 frets. Spector is 24.
Jazz neck is slightly wider at the 22nd fret.

Let's leave electronics out of this. I'm just talking about playing both instruments unplugged and getting a significantly different response.
Also, I'm simply talking about thumping. Thumb only. No popping or trying to get under the string to pull, etc.

What makes a bass slappable? What makes certain basses unslappable (strings and solid-body construction being equal? Is it just technique, or is it the instrument?

Discuss

 

That's the kind of answer I was looking for! All things being equal, some things are different!
I'll measure when I get home.

 

Thanks!
Yes, they both have the same strings DR Hi Beams 40-100. The Ibanez's are a bit older. And yes, the action is higher on the Ibby. The frets are also significantly wider and flatter (not much crown).

Time to get those feeler gauges and files and do some measuring and filing!

Thanks for the tips!

Diego