I have 3 spotlights in my bathroom and in the one the bulb keeps blowing. It works for a little while (a couple of hours) but then the bulb just seems to go
Depends if they mains voltage or not - would think that if they are 240v and it keeps blowing use a good make of lamp eg. Osram, GEC - cheap lamps never last in my experience.
If they are low voltage and are run off separate transformers get the output voltage of the transformer checked out and while you are at it, you are using the correct rating of lamp in it -should be printed on the transformer somewhere.
If they are low voltage, check that you are loading the transformer with enough bulbs. If you have a 100W transformer intended to drive two 50W bulbs but you just drive a single bulb, the output voltage will rise (a bit), but enough to shorten the bulb life.
Has it actually blown ie have you tried it in another socket? If not if could be that when you've replaced it you've slightly bent the connection it goes into. They are quite sensitive so it's easy to distort them and then they don't connect very well. If so I think you have to replace the transformer which I don't think is too difficult. This is all based on what an electrican told me when I had a similar problem, so I'm not expert myself.