I had a similar problem but the problem was, I had 10 threads quartz default number of threads in quartz properties and when I made thread dump* I found that I have 10 jobs in blocked stat, which means that I can't run any more threads.
A quick fix to this problem to increase the number of threads in the thread pool in the quartz properties.
The actual fix was reviewing my code to know why I had a 10 blocked threads.
*to do thread dump you can use kill -3 < java process number > which print the thread dump to your application standard output i.e if you running tomcat you find it in catalina.out log file
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