ProcessPoolExecutor hangs

Question

I'm trying to use ProcessPoolExecutor with the following function:

def get_ellipse_properties(a,b,mask=mask):
    ''' a -  int time frame value of a feature
        b - int feature id number
        mask- iris cube with segmentation mask'''
    feat_mask = mask_features(mask,b) #create mask for specific features
    frame_mask = feat_mask[a].data
    labels, num_labels = label(frame_mask, background=0, return_num=True) #skimage.measure function
    ellipse_features = {}
    try:
        label_props = get_label_props_in_dict(labels) #get label properties into dictionary format
        if len(label_props.keys()) > 1:
            print('More than one key found in the dictionary')
        list_of_props = [label_props[1].eccentricity, label_props[1].centroid,
        label_props[1].axis_major_length, label_props[1].axis_minor_length, label_props[1].orientation]
    #eccentricity, centroid, axis_major, axis_minor, orientation
        ellipse_features[b] = list_of_props
    except:
        ellipse_features[b] = np.nan
    return ellipse_features

This is the part of the code with the executor:

def main():
    start = time()
    pool = concurrent.futures.ProcessPoolExecutor(mp_context=mp.get_context('fork'),max_workers=30)
    results = list(pool.map(get_ellipse_properties,frame1,feature1))
    end = time()
    print('Took %.3f seconds' % (end - start))
    return results
if __name__ == '__main__':
    main()

The issue is when I try to use the executor with this function, it hangs. It doesn't throw any errors, it just freezes. I know it has something to do with my function because it worked fine with other simple functions, but I cannot figure out what exactly causes this issue. It worked well with ThreadPoolExecutor, but ThreadPool didn't speed up compared to for loop. I will be grateful for any suggestions.