add stock from plugin

hello i have this code:

$product = new WC_Product($product_id);
					
					if( !$product->has_child()){
						
						
						
						
					} else if( $product->has_child() ){
							
								foreach($prod->products->product AS $varition ){
									
									$varition_id  = wc_get_product_id_by_sku($varition->sku);
									if($varition_id != 0){
											$varition_for_up = new WC_Product_Variable( $varition_id );
									
											if($varition->available != $varition_for_up->get_stock_quantity()){
												$varition_for_up->set_stock($varition->available );
											}
									}
									
								
										
								
								}
					 
					 
					}

for some reason set_stock function is not working…
any idea?