Possible Watermelons are insect pollinated. In the absence of insects you have to hand pollinate them.

If they didn't get pollinated your fruits should have shriveled up and fell off. Since you didn't mention that i doubt pollination was the issue. But since you mentioned a greenhouse then perhaps your native bees could not get in to pollinate? Otherwise best guess is you had yours in compacted or poor soil. Don't know what else would cause your problems.

I don't remember the fruit shrivelling up, they formed very small then never grew. The leaves remained very small as well, just not vigorous at all. I remember now also I had a few long necked gourds in the greenhouse too. They grew crazy fast and very vigorous and produced many gourds however, the gourds never got even as big as a mandarin orange and eventually started to go bad. I ended up ripping them out to create more space for my long english cukes that I thought did quite well. Soil was not compacted. Was 1st year soil where I mixed poor sandy soil 50/50 with a 3 yr aged horse manure compost that was 50/50 stall shavings/manure.

Gourds are more critical than watermelons for pollination. Several day insects can pollinate watermelons, but Lagenaria siceraria is a night bloomer which needs a moth. In a typical greenhouse you will need to hand pollinate both species.

OK, good to know about the gourds. Had high hopes for them and figured they would be neat for the kids to paint on a drizzly winter day. Not sure if I'm going to even plant them this year. Man, they sure grew fast though! Bird's Nest gourds I believe what they were.