Etching using Sodium Persulfate, Is my photosensitive board too old?
It's really hard to tell from your photo, but you may have a problem with either not enough exposure time or not enough time is the developer solution.

Cover most of the PCB with an opaque material and examine the copper at one corner.  See if all of the exposed photo resist has been removed.  You can tell by gently scraping the copper - there should be only copper in that clear area.  If there is still a coating on the copper, you need to go back and fix whatever problem you are having.

It's kind of hard to offer advice.  Generally speaking, you need to make sure that you have enough expose time.  

We used something called an "Exposure Calculator", which is a test transparency with 5 or 7 identical patterns on it.  Each pattern has a different grade of neutral density filter covering it - this reduces the amount of light that gets to the PCB.  

You would expose your PCB for double the normal time, then develop and etch the board.  Whichever pattern gives you the best results tells you the multiplier factor that you would apply to your test exposure time value.  The multiplier factor ranges from 0.125 to 1.0.

The other variable is develop time.  This is a trial and error process but the develop time range is fairly wide - you only have to be close, not exact.