It doesnt mean that they are not working anymore. The fact that you are not increasing the amount of push ups will keep your bodys muscle at the same level, and if continued, will not increase or decrease. When you slowly add more pushups, say 10 more than the week before, muscle strength and mass will continue to build. The types of excersizes that will benefit you best will depend on results that you want. Etc: large biceps, more toned pectoral muscles, etc.