The struggle among manufacturers to fill open jobs is likely to continue because of the shrinking labor pool and growing need for workers with high-tech skills. Those are conclusions in a report from the National Association of Manufacturers, a trade group for manufacturers, including thousands of small businesses. In a survey in March, manufacturers said their inability to attract and retain talent was their biggest concern as it was for the previous two-and-a-half years. A quarter of those surveyed said they’ve had to turn down business opportunities because they didn’t have the staffers needed to fulfill contracts.