Do F1 hybrid geraniums give have full heads the second year

Give it time and lots of water regularly and it will! Don't forget they would have been brought up in nursery conditions in their first year when you bought them, so that would have made them bigger than they are naturally grown.

have you repotted them and or fed them recently? If the plants are looking a bit weedy I would build up its strength and then let it flower

feed and water well. if in pots repot into a bigger pot too with fresh compost. you might want to remove the first few flower buds so they are 'forced' to produce bigger flowers.