This solution WORKED for me.
I have: iPhone 7S and Jam Plus HX-P240 (3 yrs old, circa 2015)
My iPhone showed Jam speaker but showed "connection unsuccessful" when I tried to pair. I tried several fixes including resetting the speaker. Nothing worked until I followed the chosen suggestion.
1) Turned Jam Plus speaker OFF.
1) In iPhone bluetooth settings selected Jam Plus, hit the little "i" button & selected "Forget this device".
2) Turned Bluetooth OFF on iPhone
3) Turned JamPlus ON and held down the power button AND "+" button (I did this with left hand - I'm right-handed - this left right hand free to do step #4).
4) Simultaneously in iPhone settings, turned Bluetooth ON (still holding down the speaker power and + buttons).
5) iPhone discovered the Jam right away. I selected it & it connected!
NOTE:
To test I turned both off several times & each time it connected JUST BY TURNING ON. I did NOT need to hit the "+" again.