I don’t know that you’ll get an ACK per-se, but whatever is calling the Spark Cloud API will receive some sort of JSON response, even if it’s a 408 timeout (the Cloud was unable to reach your Core). You may want to see if there’s a way you can display the JSON response or log it for later viewing so you can debug a little more.
Personally, I’ve had problems with a Pebble app that I wrote to control my deck lights. It doesn’t work unless the Pebble app is open on my phone (iPhone 4S), and even then, it can be sketchy. I’m convinced it’s something to do with the Pebble <–> Phone <–> Internet and not the Spark Core itself as I wrote a simple mobile web interface to do the same thing from my phone and it never has any issues.
@ChrisLewis knows a lot about Pebble and Spark, so he may be able to comment. Everything I know about Pebble programming, I learned from his tutorials!