How to start independent, concurrently running Python processes from Java

I have a multi-threaded Java application that calls a Python program via Runtime.exec(). This works fine. I now wanted each Java-Thread to start its own Python process for concurrency.
While this is working I ran into the issue that all Python processes seem to restrict themselves to a single CPU and thus each process only uses part of the CPU to run. In top I can see my n Python processes.
With n=1, the process uses 100% CPU.
With n=2, both processes use approx 50% CPU.
With n=10, all processes use around 10% CPU.

In htop I can see that only two CPUs are used: One for Java stuff and the other for the Python stuff.

I thought that running multiple Python processes would allow them to run completely independently from each other.

Ideas and hints? Thank you!

EDIT: Here is the code that leads to the creation of the Python processes. It's not a minimal example. I would create one if this isn't clear enough.

        ProcessBuilder builder = new ProcessBuilder(new String[]{"-u", "-c", script});
        process = builder.start();
        errorStreamConsumer = new
                ErrorStreamConsumer(process.getErrorStream(), options.getTerminationSignalFromErrorStream(), Thread.currentThread());
        errorStreamConsumer.start();
        log.debug("Started process with arguments {}", Arrays.toString(arguments));
        BufferedInputStream bis = new BufferedInputStream(process.getInputStream());
        BufferedOutputStream bos = new BufferedOutputStream(process.getOutputStream());

where script is the complete Python script as a String (NOT a file name but the actual Python code) the ErrorStreamConsumer is a thread printing out the error channel. The communication with the process runs over the the bis and bos input- and output streams.

I do this for each Java-Thread. And it works fine. Except that the Python processes seem to share a single CPU.

First of all: Thanks for everyone who thought about my issue.

While I am a bit ashamed of the true nature of my problem I feel I should share what the issue was.

I ran my program with SLURM, a job scheduler which is also able to restrict CPU use. In my case it restricted my program to use a single CPU. Thus, more CPUs were never used by definition.

Now that I realized that I have no issues with Python concurrency at all.
To be more concrete:
The -c parameter to pass the script to the Python interpreter is not an issue here.
Also, the idea of kiran does not apply here. I have one Java Process and multiple Python processes started from the Java process and now they all consume as much CPU capacity as I would give them (I just had 3 x 300% of CPU usage from the Python processes).

So the real problem here was my assumption that this would be a Python issue. I apologize for this and will try to be clearer about my problem in the future.

Threads share the cpu of the parent process. If we have 5 threads that doesn't mean we can make use of all 5 cores we got, each thread will the share the cpu/core of the main parent process.
In your case 10 threads were sharing 100% cpu so you got 10% to each.
Now each thread is running a python code with 10% cpu hence that is the computing power you got for python.
I suggest you to do multi processing instead of multithreading. Like, each java process starts a python process and you can deploy multiple instances of java.