RollingFileAppender not rolling logs

My log files are not getting rolled as expected. The RollingFileAppender is
given below. My Application.log gets overwritten when the max size is
reached. I have already done the following and haven't found the cause of
the problem yet:

1. changed application.log to app.log in case two instances were writing to
the same log file, but app.log also got overwritten.
2. changed maxfilesize to lower amount (to 15KB from 15MB). It gets
overwritten after 15KB.
3. Compared with other working log4j.xml files and haven't noticed anything
significant.
4. Restarted the server and no effect.

Has anyone faced this problem before or have ideas how to track it down?

<appender name="xxxRolling" class="org.apache.log4j.RollingFileAppender">
<param name="Threshold" value="DEBUG"/>
<param name="File" value="d:/logs/app_logs/project/Application.log" />
<param name="MaxFileSize" value="15KB" />
<param name="MaxBackupIndex" value="15" />
<layout class="org.apache.log4j.PatternLayout">
<param name="ConversionPattern" value="[5.st]%d[5.DT]%-5p[5.PRI]%X{USERID}
[5.NDC]%t[5.THD]%m[5.MES]%c[5.CAT]%X{enterpriseSessionId}[5.LOC]%n" />
</layout>
</appender>

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