Pioneer Turntables won't let the arm go down onto record

[Steve T]

Hi, I have a Pioneer PL-910 and a PL-L550 turn tables. Both have a button  and indicator light  that lights when the arm is up in the elevated position. When I push the start button on either turntable the arm travels over the record but does not descend onto the record and the indicator stays lit. Both are 25 years old but the PL-L550 hasn't been used much and has been in storage. Can someone advise as to this problem?

Thank you

Steve T