Check your chain for wear. If it s very worn it may not engage the teeth in the new pulleys correctly. If it quite stretched (worn, acutally) it might be prudent to replace the chain and freewheel before the chainwheels are excesssively worn as well. It is possible that the new pulleys will wear into the old chain but it is a destructive kind of wear, not "break-in". I don't trust chain gauges, measure 12 links with a good steel rule and replace the chain if they measure over 12 1/16". If the freewheel skips on the new chain it will need to be replaced as well.