IPhone not connecting to computer -flashing "trust/don't trust" on iPhone

I am trying to connect my iPhone 7+ to my Macbook Air to download some photos. When I connect via my USB port, the connection seems to flicker. In Photos I can see my iPhone for half a second then it disappears and reappears a few seconds later etc. etc. The "trust/don't trust" this computer briefly flashes on my iPhone screen but never long enough for me to actually click it. Running Mojave 10.14.3. Can someone help?