All,
I was hope full to get some feedback how to best proceed.

Building info
The building is a commercial building that has a cooling tower out back (open loop with plate heat exchanger) and has a closed loop of glycol that runs through the entire building through plastic pipe. Throughout the building, we have heat pumps of various sizes (Most of them 2-3 ton units) that are ducted to the various offices.
We have service checks on the system every 6 months, including checking the PH of the loop and adding alga-cide to the cooling tower.
We have roughly 18 heat pumps in the building

Building history
The building always has had HVAC issues. Condensate leaks were very common due to clogged condensate lines. Occasional glycol leaks also happened from time to time. And on average, about once a year we would have a system failure (Usually the cooling tower would get clogged, turned off, trip breakers, etc...) causing the building to get very warm.

Remodel info
We went through a remodel of the building, we changed the configuration of offices, we added three new entrances to the building, we removed some old units, and added some additional units. Also, we installed a brand new BAC cooling tower.
Also, just recently we cut down alot of trees around that building that did provide some shade on the building.
During the remodel, we drained the main loop once to change some plumbing. We also had a pipe break in an office which drained the loop again.

After the remodel
Now the remodel is over, we have heat pumps that trip out due to high head pressure everyday. On a good day, only one will trip. On a bad day, 10 of them will trip.
Main loop temperature is around 84 F on a day with 90 F outdoor temps.

Suggestions
Several suggestions have been made regarding the cause of our problems with the heat pumps tripping on high head pressure.
Not enough water flow to the units, The HVAC balancer showed up and did testing and found three units that were not getting enough water flow. These units have been tripping often, so that is no suprise. But the units that are getting good water flow, are also still tripping.
There has also been talk that our heat exchanger is not exhausting enough heat. I measured the temps today during 85 outdoor temps:
Water temp from tower = 67 F
Water temp to tower = 75 F
Water temp from building = 79 F
Water temp to building = 72 F
The plate heat exchanger is 20 years old (Same age as the building) and as far as we all know, never been taken apart.
Because of the leaks we have had in the past replacing the plastic pipe with copper pipe has been suggested. Problem is the cost of the copper would be very expensive.

There has been talk of acid flushing some of our heat pumps to hopefully to clear the clogs and increase the overall water flow.
My question - Is it worth the time, money, and labor to flush the heat pumps or should we simply replace the heat pumps with new ones?
The heat exchanger as far as everyone knows has never been taken apart. Looking at the temps listed above, I believe the heat exchanger might have some build up in it.

Suggestions
Based on the information presented above, what would you all recommend?
Start replacing the heat pumps that are problematic and replace our heat exchanger?
Questions/Comments/Concerns/Other Suggestions?