Yes, as I wrote earlier, it could be a damaged connector or a damaged jack, or even just dirty, or a motherboard issue. Given what you've tried, I suspect the motherboard. But, try these steps: Parts-People has genuine Dell adapters for your system.
Recently after some BIOS updates I get the warning that my AC adapter may not be suitable.Battery charging level is actually kept, but not increasing with AD adapter connected.This means bending pin etc. can be excluded as a cause.Even with a new battery replaced the situation does not change --> no battery issue.WIth AC adapter disconnected battery level is reduced, for example to 78%.After AC adapter connection again chargiong level is kept again to 78% this time.Once the battery level went down to below 10% without AC adapter connection, and the charging worked to go up to 100%.Then again the same situation above comes.Charging level is always kept with AC adapter connected, and charging is started if the charging level goes down to below 10%.Thus I have a doubt on BIOS.
Vostro 1550, Battery charging level is kept, but not increasing with AD adapter 
connected