Upload problems with Arduino Pro Micro 3.3V

I have just purchased the Arduino Pro Micro 3.3V

the Drivers are correctly installed and the add on for the Sparkfun boards - Device recognized in device manager - all ok

The first time I tried uploading the blink sketch it worked great - LEDs blink as expected.

All subsequent times the upload does not work - the upload starts and the 2 LEDS start to blink very rapidly. Eventually the upload fails with error…Couldn’t find a Board on the selected port. Check that you have the correct port selected.

The LEDs continue to flash rapidly until I reset the board - then it goes back to normal - device recognized.

I can sometimes upload sketches by hitting reset and putting it in bootloader mode - but this method is flaky and difficult.

It absolutely refuses to upload sketches when NOT in bootloader mode.

I am using version 1.8.1 of Arduino and Windows 10

Any help much appreciated

Thanks