Hi Uma, if you add too much sorbent or chelating agent as in your case, then the molecules of this material hide each others active binding sites. If these sites are not uncovered then metal ions can not be binded. Here is the reference to the paper where authors made similar observation to yours:

"Removal and recovery of heavy metals from aqueous solution

using papaya wood as a new biosorbent''-Asma Saeed a, M. Waheed Akhter b, Muhammed Iqbal, 2005

Yes phenols and flavonoids show metal chelating activity, Chelation therapy

reduces iron-related complications and thereby improves quality of life and overall survival in patients. Main reason is these compounds do not bind in a proportion of solute concentration but inversely show  non binding or deposition of either negative or positive ions in the active site region or fill them cloudously, Feijoa sellowiana, Sambucus ebulus, Pterocarya fraxinifolia, Mellilotus arvensis, Salvia glutinosa, Diospyros lotus possess phenols and flavonoids which show metal chelating activity or iron chelating activity. 

[A] First how to measure Metal chelating activity[ See REFERENCE [I]

The chelating activity of the extracts for  any metal ion [say ferrous ions Fe2+] is measured according to the method of Dinis et al.{ SEE REFERENCE [II]}, with slight modification. To 0.4 ml of methanol extract, 1.6 ml of methanol is diluted and mixed with 0.04 ml of FeCl2 (2 mM). After 30s, 0.8 ml ferrozine (5 mM) is added. After 10 min at room temperature, the absorbance of the Fe2+–Ferrozine complex was measured at 562 nm. The chelating activity of the extract for Fe2+ is calculated as :

Chelating rate (%) = (A0 - A1) / A0 × 100

Where A0 was the absorbance of the control (blank, without extract)

and A1 is the absorbance in the presence of the extract.

[B]Explanation of decreasing chelating rate with increase in concentration :

According to Beer-Lambert law: 

A=ε bc

As both ε and b are constant for the same instrument, the value of A1 in equation [i] will increase with the increase in concentration AND THE DIFFERENCE BETWEEN (A0-A1) WILL DECREASE AND SO THE CHELATING RATE DECREASES.

[I] Int J Pharm Pharm Sci, Vol 4, Issue 2, 567-574(2012)

MITALI GHOSAL, PALASH MANDAL

[II] Dinis TCP, Madeira VMC, Almeidam LM. Action of phenolic derivates (acetoaminophen, salycilate, and 5-aminosalycilate) as inhibitors of membrane lipid peroxidation and peroxyl radical’s scavengers. Arch Biochem Biophys 1994;315: 161.

Whether metal chelating activity standard curve is decreasing trend or increasing trend? using EDTA STANDARD

it is low dilution activity