How are you loading the cartridges into the magazine? The No. 4 Mk. I fires .303 British which, as you probably already know, is a rimmed cartridge. The Lee-Enfield does not have an interruptor like a Mosin-Nagant does, so it is imperative that the rim of each cartridge is in front of the rim of the cartridge below it in order to prevent rimlock. Improperly loading cartridges into the magazine is a common mistake of people used to shooting rifles chambered for rimless cartridges like 8mm Mauser, 30-06, and .308 because rimlock is impossible with a rimless cartrdige.