The Swedish empire controlled the Gold Coast in 1650-63 and New Sweden in present-day USA in 1638-55. Why did Sweden fail to hold these possession in the apex of its great power era?
The failure of Swedish colonies such as New Sweden was due to the fact that they were trading posts, rather than true settlements or colonies.
New Sweden was attacked by the Dutch, and barely managed to resist. The stronger English naval forces conquered New Sweden.
You could argue chicken and egg, did strong home country support lead to strong settlements or vice versa? But in the end, only the English had both in North America. At the time of the French and Indian War, the English had 1.6 million "colonists" in North America; the French had 80,000. Small wonder that the English beat the French, after a long (seven years') war.
It was about a century earlier, but Swedish settlers numbered in the hundreds; English settlers in the tens of thousands. Meaning that the Swedes had less of a chance than France.
A similar thing happened in the Swedish Gold Coast, except that the Swedes were beaten first by the Danish, and then the English, who also had "only" trading posts. In any event, Sweden lacked true colonies there also.