Well that sux and I'm sure you are bummed. Couple things, did you use cover crop of WR or WW? That helps. Was the soil firm but not hard before spreading small seed like clovers and brassica? It should be. No more than inch settlement when you step on it with your size 11. Did you pack seed into soil with ATV, tractor , packer, feet, after spreading? That helps. Regardless of soil test with tilling did you add fert, especially N, such as 19-19-19 and urea? Another plus.
Good news, not all is lost. Overseed with WR and/or WW next couple weeks before rain. Also the clover is slow to start, but by next spring will probably take off. Frost seed a white and red clover early to late March next year. Add fert of 0-20-20. Bet you get a decent plot. Good luck. We all have failures. The last two years have been rough planting due to inadequate or poorly timed rains. I had to overseed some of mine after 12 weeks of no rain after planting. Always have you options.
And finally, it gets said all the time, have an exclusion cage, it can seem a plot failure when in reality can be kept browsed before chance to let plants take off.