Block convolution method in Matlab problem not repeatable

I am performing Block convolution method in Matlab. I am planning to use this in the future for my main project as well. Currently the entire code takes 20-40 s to complete the execution. But, sometimes for certain inputs the Matlab doesn’t execute the complete m script and waits infinitely. I have to later kill the task or use Ctrl break. But, if re run the same code without changing the inputs, the code executes properly. What might be the problem of the cause?