I don't think it's a systems issue. It sounds to me that the OP has had numerous employments, so I am presuming that he was a member of their pension schemes (rather than having a personal pension).
Because these are Employer sponsored arrangements & are held under trust, you cannot start contributing to them when you join a new employer.
If this is the case, then your pension provider is quite right not to allow you to start contributing to your previous policy, as this may affect the Revenue Approval of the whole scheme.
The employer/trustees of your new scheme cannot make contributions to your previous employers scheme.
PRSAs are more flexible however because they are not governed by trust law, rather they are an individual contract between the contributor & the provider. So you can indeed stop & start premiums when you want.