Interest rates were falling, the economy was strengthening, demand was high for bonds that pay tax-free income and supply was relatively low. Municipal bonds are producing less income than a year ago because their yields key off Treasury rates, and the yield on the 10-year Treasury note is below 2 percent. [...] the improved job market means most economists expect the Federal Reserve to raise short-term rates later this year. States, cities and other local governments are issuing bonds at a faster pace this year, and when the supply of anything increases, that can push prices lower. Just look at what's happening to oil, where a buildup in supplies has caused its price to more than halve since last summer. Governments are taking advantage of cheap borrowing costs to replace higher-cost debt and pay for new projects. Individual investors make up the bulk of the market for municipal bonds and they're pouring in billions of dollars each month. A larger workforce means higher collections of income taxes, sales taxes and other revenue. Managers have concerns about finances in some high-profile areas of the market, including Illinois and Puerto Rico, as well as bonds from government that are reliant on oil-related revenue.