When you pull a tube from the socket is there a physical pulling resistance or does the tube pull out easily with little effort on your part?
If there is good contact pressure, therein a strong physical pulling resistance
it may be the tube pins and or the female contacts of the tube sockets are dirty and or corroded. Usually just pulling the tube from the socket and reinstalling it a few times will clean dirt and or corrosion from the tube pins and tube socket contacts.
Did you buy the amp new or used? If used you probably have no way of knowing if the previous owner used any contact enhancers on the tube pins and socket contacts. Once that stuff dries hard from the high heat of the tube it is next to impossible to clean it out of the female contacts of the tube sockets.
If the tube socket contacts have lost their memory retention you can use a women’s hat pin, or something similar, to reform the contact for a tighter fit against the tube pins.https://www.amazon.com/Hat-Pins-6-12-Pkg-Nickel/dp/B001682GGG
You will readjust the tube socket female contacts from the top of the sockets. Gently wedge the pointed end of the hat pin between the tube socket wall and the outer edge of the contact near the split in the contact. Apply a little pressure on one side of the contact split, then the other side of the split. Use care not to over do it. It doesn’t take much.