Items occasionally do not appear

[MicroSTM]

I designed a game where a diamond appeared at a fixed location and there was a reward for collecting it. When I run the game for a long time, the diamond did not appear occasionally, and consequently, no reward was given even when the agent reached the location. The decorator is specified as follows:

    <FlatWorldGenerator destroyAfterUse="true" forceReset="false" generatorString="3;7,220*1,5*3,2;3;,biome_1" seed=""/>
    <DrawingDecorator>
      <DrawCuboid type="air" x1="-5" x2="6" y1="45" y2="226" z1="-6" z2="12"/>
      <DrawCuboid type="stonebrick" x1="-5" x2="5" y1="45" y2="226" z1="-6" z2="11"/>
      <DrawCuboid type="air" x1="-3" x2="3" y1="45" y2="226" z1="1" z2="6"/>
      <DrawCuboid type="stonebrick" x1="-3" x2="3" y1="60" y2="226" z1="-6" z2="6"/>
      <DrawCuboid type="sandstone" x1="-3" x2="3" y1="45" y2="45" z1="1" z2="6"/>
      <DrawCuboid type="stonebrick" x1="0" x2="0" y1="45" y2="52" z1="6" z2="6"/>
      <DrawCuboid type="stonebrick" x1="-2" x2="2" y1="45" y2="52" z1="5" z2="5"/>
      <DrawCuboid colour="BLUE" type="stained_hardened_clay" x1="-2" x2="-1" y1="45" y2="45" z1="6" z2="6"/>
      <DrawCuboid type="stonebrick" x1="0" x2="0" y1="49" y2="49" z1="4" z2="4"/>
      <DrawBlock type="torch" x="0" y="50" z="4"/>
      <DrawCuboid type="stonebrick" x1="-2" x2="-2" y1="49" y2="49" z1="4" z2="4"/>
      <DrawBlock type="torch" x="-2" y="50" z="4"/>
      <DrawCuboid type="stonebrick" x1="2" x2="2" y1="49" y2="49" z1="4" z2="4"/>
      <DrawBlock type="torch" x="2" y="50" z="4"/>
      <DrawCuboid type="stonebrick" x1="-3" x2="-3" y1="49" y2="49" z1="6" z2="6"/>
      <DrawBlock type="torch" x="-3" y="50" z="6"/>
      <DrawCuboid type="stonebrick" x1="3" x2="3" y1="49" y2="49" z1="6" z2="6"/>
      <DrawBlock type="torch" x="3" y="50" z="6"/>
      <DrawCuboid type="stonebrick" x1="3" x2="3" y1="49" y2="49" z1="4" z2="4"/>
      <DrawBlock type="torch" x="3" y="50" z="4"/>
      <DrawCuboid type="stonebrick" x1="-3" x2="-3" y1="49" y2="49" z1="4" z2="4"/>
      <DrawBlock type="torch" x="-3" y="50" z="4"/>
      <DrawCuboid type="stonebrick" x1="3" x2="3" y1="49" y2="49" z1="2" z2="2"/>
      <DrawBlock type="torch" x="3" y="50" z="2"/>
      <DrawCuboid type="stonebrick" x1="-3" x2="-3" y1="49" y2="49" z1="2" z2="2"/>
      <DrawBlock type="torch" x="-3" y="50" z="2"/>
      <DrawCuboid type="stonebrick" x1="0" x2="0" y1="49" y2="49" z1="1" z2="1"/>
      <DrawBlock type="torch" x="0" y="50" z="1"/>
      <DrawBlock type="torch" x="0" y="48" z="6"/>
      <DrawItem type="diamond" x="1" y="46" z="6"/>
    </DrawingDecorator>
    <ServerQuitFromTimeUp description="out_of_time" timeLimitMs="10000"/>
    <ServerQuitWhenAnyAgentFinishes description=""/>
  </ServerHandlers>