I agreewith KNott but another possibility is the expiry date on the sunblock- sunblocks can "expire" so you may want to check- where I am ( Canada) I understand that you shouldn't leave them thru the winter in cars, boats etc because the freezing temps ruin them.
Also are you putting it on "thick'" enough. Apparently when they test the spf on sunscreens they test it using a whole tsp per arm leg etc- most people just put it on too thinly so the spf doesn't match up to the claims.
Hope that helps We all need to protect our skin!