There are several possible reasons:
• The song has not enough information to properly identify it. Jiwok uses the artist's name, the song's title and the album's title to identify the song. If the song can't be identified, Jiwok can't find its BPM and ignores it.
• The available information has been entered manually at some point and doesn't match the actual information that was present on the original CD. The mismatch breaks the identification and the song can't be used.
• The song has been bought on iTunes at the time Apple used digital rights management (DRM). The protected songs can't be used from outside of iTunes (or Music on iOS). There are several legal ways to unprotect these songs, easy to find on the internet.

Each music-providing service (iTunes, Spotify, others that may appear later), offers its own application to do this, and doesn't necessarily allow a third-party application such as Jiwok to modify its playlists. Jiwok being sort of a “client” for these musical contents, it was a logical choice to not try to replace these specialized applications to handle their own contents. It is rather simple to move from an application to another one, and it's better than reinvent the wheel and make the Jiwok application overloaded with redundant features.

This is Spotify's choice: only users with a Premium Spotify account can access Spotify from a third-party application. This is not a limitation specifically related to Jiwok.

Perhaps your active playlist has too few songs within certain BPM ranges. Even if your playlist contains 50 songs, if only 5 of them match the “relax” BPM range, as soon as the training session needs a “relax” moment, the song will be selected only among those 5 songs. Don't hesitate to activate more than one playlist together to make the available music span larger and create sessions with more musical surprises. 

If no song in the active playlists matches the BPM range needed by the training session, the application falls back to the “Essentials” pack, which can’t be deactivated precisely for this reason.

Each workout indicates one or more paces to follow during your session. They are shown as % MHR (as a percentage of your Maximum Heart Rate).
Heart Rate (HR) is the number of heart beats per minute.

To work most effectively, it is important to reach your maximum heart rate (MHR).
As you can see from the various sessions of Jiwok training, your progress and movement are guided by this measure, so it is good to know!

To do this simply, evaluate your form during a run of 20 minutes at an endurance pace (where you can talk without being breathless) and finish the effort by accelerating quickly for at least 1 minute going as fast as you can for the last 20 seconds.
This is probably the simplest and least restrictive method, but not the most accurate or recommended over the age of 40 without notice from your doctor.

The latest idea is to estimate the 220-age method for men and 226 for-age women. This calculation, however, is also less precise.

A basic guideline is this:

Running from 65-75% MHR: You must be able to run without being out of breath when speaking, if you are breathless, you must slow down.




Running from 76% to 85% MHR: You must be able to run fast and if you try to talk you are out of breath.

Running from 86-95% MHR: You are exercising full out with maximum effort, almost at the limit of your capabilities.

Courir de 65 à 75 % FCM : Vous devez pouvoir courir en parlant sans être essoufflé, si vous êtes essoufflé, vous devez ralentir.

Courir de 76 % à 85 % FCM : Vous devez pouvoir courir rapidement et si vous essayez de parler vous êtes essoufflé.

Courir de 86 à 95% FCM : Vous êtes presque au maximum de votre capacité