There are a list of reasons that your products may not be showing in the front-end system.
There may be no pricing applied to product (i.e. price may be set to $0.00).
Just go into your product through Menu --> Category --> (Click on the magnifying class labeled Products next to the category this product belongs to) --> (Click onto the pencil icon labeled Edit). Then once you are on this page, press on the second tab from the top-right of the page listed as Sizing / Pricing. This is where you can set the price for the product which will fix the issue.
Could be a timed category or product, which means the category/product disappears from front-end.
Make sure that your product availability is set to YES every day of the week if you wish for that to be the case. To check or change this go into Menu --> Categories --> ( Press the pencil icon under the Edit column) --> Go to the last tab at the top right as shown below.
Lastly it can be that no sizing attribute applied to the category.
To fix this issue, create a sizing Attribute Category, then create the different Attribute Items within it that are applicable to the product.
Click on the magnifying icon in the column Attribute Items
Click on the pencil icon in the column Edit
Make sure the items and the category are set to active (Green).