Without pictures two things come to mind;
1) the fixed sprocket is bigger and/or you have taken too many links out and it doesn't fit. In this case you need to add more;
2) you're not putting the chain on correctly.
Listen to Prancer - youtube, parks, and ehow.com. Watching a few videos will explain more than an message board.
Good luck.
@hugo7 started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
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Without pictures two things come to mind;
1) the fixed sprocket is bigger and/or you have taken too many links out and it doesn't fit. In this case you need to add more;
2) you're not putting the chain on correctly.
Listen to Prancer - youtube, parks, and ehow.com. Watching a few videos will explain more than an message board.
Good luck.