Watch GPS doesn't lock at all sometimes

So, yesterday I went for a run and started my GPS activity in the same spot outside my building in London as every time I go for a run (5 times a week). For the second time the watch couldn't acquire a lock after several minutes. I'm on FW 4.0, the first time happened on 3.9. I use GPS+Glonas.

I was freezing, so after a while I just hit start and went for my run. It was a bit cloudy but not too much. There are buildings around that spot but not very high. Sometimes the watch locks in seconds, others in minutes...

Is this what I have to expect from a GPS watch the cost more than 500£? Seriously...? I'm thinking to sell it and buy a 935 which many say is more reliable than this one.

I'm very disappointed about this watch.

Aurelie