Starting Problem Troubleshooting

I been having problems starting my motorcycle. I purchased a battery tender and that helped a bit. Afterward, I started to commute everyday for approximately 12 miles roundtrip and on my fourth day, the bike could not start. The afternoon of that day, I tried charging the battery, and testing and disassembling the starter motor and relay to no avail. Since I purchased a "just in case" replacement battery from WalMart, I charged it and replaced the existing battery, and baamm! starts with no effort. Apparently, due to my short commute, the charging system does not have enough time to charge the battery. Hence, the battery tender is now installed nightly.