There is two main reasons why you "can't get it to stick"...

First is most pots have a plastic or oil coating on the metal can to prevent rust. This ultra thin coating will often just melt away when soldering but a quick rub with some sand paper or a scratch with a sharp knife will certainly speed up the process.

Secondly, and more importantly, any failure of the solder joint is most probably due the items being soldered did not get to the right temperature before the iron was taken away. On a pot it means the metal can at the point being soldered simply did not get hot enough. This is where the the "trick" comes in, being getting the one spot we want soldered to be hot enough but also to not put so much heat in that it destroys/melts the more heat sensitive parts of the pot. The little $10 soldering iron units of 15W to 25W are only just big enough to do the job by someone with experience. Ideally an iron of between 35W and 65W will make the task easiest. Over 65W will require a skill set that requires considerable delicate practice as there will be a LOT of heat available, often way more than is needed.

For soldering old and corroded pots it is probably best for a novice to take the job to a specialist.

Side note, The task will be simplest and less hassle for a novice soldering student when using 60/40 resin cored solder. Newer Lead free solders are much more critical of temperature and also require notably higher iron temperatures than 60/40 types. If RoHS compliance is needed then get the Lead free solder stuff otherwise stick to the resin cored 60/40 or similar Tin/Lead ratio solder as supplied in all PBG kits or available at your local electronics store...