AGi32 v20 will use the same custom symbols that you have been using in older versions of AGi32. However, AGi32 v20 has some new filters that may make it seem like the custom symbols are "missing".
There are 3 important areas to check when looking for symbols.
1. The Folder dropdown. By default, this will list the symbols in the Factory Symbols folder. Your custom symbols will be located in a different folder, likely a folder called "Custom Symbols".
2. The Category Filter. AGi32 v20 also includes a Category filter to help filter symbols based on their function. Using Smart Symbols will apply a filter here to limit it to the category of the previously selected symbol. Your custom symbol may not fit this category, so adjust it to "All" to show all symbol categories.
3. Drawing Symbols. Your custom symbol may be a drawing symbol. This is different from a Render symbol. If you cannot locate your custom symbol, please check the Drawing Symbols as well. More detail can be found in this article.