Need more info. Old 3 1/2 gal toilet, newer 1.6 gal toilet, etc??? On any toilet, check that the tank water level is at the mark in the tank(back wall of the tank, a line or decal). On an older 3+ gal toilet, the siphon jets could be clogged with calcium deposits. Drain most of the water from the bowl and fill with CLR or similar to dissolve the deposits(let sit overnight). Scraping around the siphon jet opening also helps. Those deposits also develop up in the bowl elbow. This can also happen with newer toilets. If you happen to have a 1.6 gal toilet that was new when the new law was passed(about 1995-1998), junk it, it will never work right.