Need help with MODS not activating, please

You need sets of 4 for speed.

Speed, offense, crit damage work in sets of 4. Health, defense, tenacity, potency, crit chance need sets of 2.

Fantastic, thank you.

Speed, Offense and Crit Damage all need sets of 4. The other types just need sets of 2.

If you tap/click on the 'Set Bonuses' area below the mods you will get a popup window with more information.

Also, to be clear, it's just the 10% speed bonus that's not activated. The primary stat and secondary stats from that mod are still being applied.