This is a great tool for helping kids to learn maths, however like most things IT related it can go wrong and the solution isn't sometimes obvious.
Mathletics uses a lot of "flash" content. Although flash is great for animated content, there are known security vulnerabilities. So it's not advisable to allow flash to run from sites you don't trust.
As such all modern browsers disable Flash by default, the idea being that you can enable flash for sites that you trust. N.B. some don't work well with flash e.g. Edge
Not all of the content from Mathletics comes from the same source address on the web (URL). So the best way to make sure you don't hit any flash problems is to allow flash for all of the current mathletics sources and any future ones as well.
Its different for each browser so click on the following help page from Mathletics, choose your browser and follow the instructions.
If this doesn't solve the problem you are having it could be a more general incompatibility so try having a look at the general help page. Click on the link below.
Sometimes tablets can have problems with web pages, a way to get round this is to use the Mathletics app for tablets.