Garmin "create a route" function

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I have a Garmin 405 (and have had a 305 in the past), but seem to have an issue with the "create a route" function.  I was in Barcelona recently and created a route from the hotel in advance of the run and transferred it to the watch, but when I started running the route the watch didn't seem to want to commence the route even though I thought I was very, very close to the start point.  After 2 or 3 jogs up and down the pavement, the watch still couldn't seem to find the start.  Even when I knew I was on the route (i.e., maybe 1/4 mile up the road from the start point), the watch still told me I was off route, and was indicating I should turn around and go back to the start point.

I ended up ditching the route and just running to the sea front and back, but it kind of defeated the object of setting up the route in the first place.

For info, my hotel was in a fairly built up area (tall buildings around), but I've had a similar issue in open spaces too in the past, using the 305, but I suppose it could have been a GPS issue, but I'm not convinced by that.

Any advice on this?  Is there a setting somewhere I've missed?  The reason I'm asking is that I'm off to Budapest soon and would like to get a couple of different pre-planned routes in as opposed to just running back and forward along the river.

I hope this all makes sense!