Poorer countries with less developed markets are forced to open their domestic markets to foreign countries and competitors who then completely outcompete and dominate the local offerings. This prevents local business from developing and many existing ones go bankrupt in the face of foreign competition. This tends to leave the local market largely undeveloped in terms of domestic companies, and results in money flowing out of the country to foreign shareholders.
Another problem is that so-called "free" trade isn't totally free.
Some of the places that espouse free trade, like the US and Europe, don't practice what they preach. The most egregious example I can think of is agriculture subsidies, which effectively prevent (for example) African farmers from selling their produce in those markets while additionally making produce (like poultry) from those countries cheaper than local offerings, which again bankrupts locals.