Chain on front cog slipping

[Cycling Dan]

So as I move the gearing so it will move onto the big ring on the front it slips down and off. It wont get onto the big cog it the chain slides next to it not touching the teeth. It does this between rings 1-4 on the back cog.
I did have a problem a few weeks ago where it would jump off and over and it was changed for me by the bike shop. However after a few weeks I have found it cant get onto the second ring and it slips off? Do I take it back to the shop or is there an easy fix?

 

[gwhite]

I'm having difficulty deciphering this.
The chain-set at the front has "rings". On the back, the block has "cogs" or "sprockets". Your problem seems to be the chain coming off a chain-ring at the front...right? If so then the "H" screw would need adjusted. This sets the limit for the chain to move outwards to the large ring. If this screw is wrongly adjusted then the chain will not climb onto the large ring (unscrew screw a quarter of a turn at a time until it does so) or alternatively the chain will climb and fall off the large ring on its outside (screw inwards quarter of a turn at a time).
If this isn't your problem then re-frame your question using the above terms.

 

[MrJamie]

Best find a youtube guide, they're pretty simple to set up. Make sure the mechs positioned correctly, you can check with your hand if the mech can move over far enough to change to the big ring, if not the limit screws preventing it, if its free to move it probably just needs a touch more tension in the cable to pull it onto the ring. If youre going to take it to the shop anyway, you might aswell have a go fixing it yourself first