that's a standard ratio in EU, (British) friend in Paris (doing masters) sent he claims a thousand applications and got three replies,
British friend in Osaka sent ten requests and received ten interviews (I think the Japanese are just more cultured, more polite, sensible etc etc)
he's now got a job working on the Japanese side of the OBOR one belt one road project...
looks like you need to send out another five hundred letters, or do a post graduate certificate in education and use your engineering skills as a maths teacher (another mate did Biology, then masters looked for a PHd for a year, with no luck, so was seduced by a very large welcome to teaching maths grant)
either way, keep going!