Excuse a daft question, but have you bled them?
If that doesn’t work, it may be a case of rebalancing the system between the radiators. We had a problem for years where the radiators at the ‘end of the line’ weren’t heating. Several engineers suggested a power flush would do the trick. Did that, didn’t work. Got LMS gas services in, they rebalanced the system for a fraction of the cost of the power flush, and everything now works perfectly and has done for several years.