I've also been instructing for just over three years, didn't get a sniff of an interview for then first two years, in the last year i've had three and am currently swimming in the BACon holding Pool. PPL CFI approaching 1200 hours as I seem to be flying my desk a lot recently and don't always have the time to do too much flying. Just keep your head up mate you will get there in the end. Make sure that your CV and covering letter are up to scratch if you haven't already that might make all the difference.
Hope this helps,
Han.
