Coin Shooting in bark

Noticed an interesting phenomenon with my Tracker IV today. Coin shooting local park, discriminate at 12 o'clock where it rings for goldie's, which it did, went into bark chip covered play area relying on someone losing a coin on the swing or bars and nothing...
Came back a while later and set the detector to All Metal and subsequently found a dozen coins.
Is this a common thing with fine bark mulch, there had been some rain overnight
Thanks in advance

Did find a solid gold ring in a play away .

Am thinking that your discimination is really limiting the depth of the tracker, vs being in deeper all-metal mode. Used to have a similar problem with the Ace 250 not being able to hit on coins, having to resort to pinpoint mode (poor man's all-metal mode) to enable the coins to be heard.

Were the coins very deep when found in all- metal, or did the detector simply block them out for some reason.

The coins weren't more than 3cm down. I figure it must be something to do with the damp bark adversely impacting balance.

With disc at 12 oclock the detector may still be responding to goldies but depth is reduced when compared to All Metal mode. As the disc is wound up, ALL metals begin to be rejected and at 12 the dollar coin has to be closer to the coil to be detected. Also the tracker has a fixed ground balance setting...It is NOT an Auto ground tracker.... Contrary to what you may believe.
The fixed ground balance setting also limits the depth at which a target can be detected and is why the Tracker IV is not good over wet beach sand and high iron mineralisation.