Some areas are obvious targets for hoards of detectorist, and the beach comes close to the top of the list for easy detecting with a relatively small amount of junk, hence many of the easier coin targets are cleaned up on a daily basis, especially during the warmer months. Beaches are often hotly contested for dry sand finds and recent drops, get there at the wrong time of day and you will simply miss out (depending on how thorough they are). Some people sit in their cars to pick out the most lucrative and frequented spots, and as soon as the beach is vacated, they hit it hard.
Beach conditions also play a major role in whether you will find anything of worth or not, particularly if you intend to hit the wet sand, tidal flats, rocks, gutters/cuts, or shallow water. Some days you can drive down to the beach, and just one look is enough to make a decision on whether it is even worth getting out of the car for a go. You really do have to keep constant tabs on the sand movement, ideal tides and weather conditions, and what sort of erosion is evident to give yourself the best chance.
Some older sites like abandoned schools may have also been hit hard previously by those who had done some research, whereas some more obvious locations like tennis courts can be simply overlooked or dismissed at times. You simply don't know till you get out of the car and give it a go.
I used to work over a park with an older tennis club situated at one end, and it proved to be very lucrative for pre-decimals. It was tucked away in suburbia, and somehow manged to escape the attention of most detectorists. The park section was relatively new, but the tennis courts dated back to the 1940's, so it was always going to be prospective for pre-decimals.
Bottom line is that you will always have days filled with disappointment, where you often feel more like a garbage collector than a detectorist searching for valuables. In a way I have sort of given up on the idea of looking for spendables, simply due to how often my local spots are cleaned out (thinking after community events, tot lots, and most local parks). Deeper pre-decimals can be more of a challenge to find, hence can still be regularly found by testing areas and doing a little research (and sometime by blind luck).
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